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^JAWAH  AGRICULTURAL  EXPERIMENT  STATION, 

Kl^D  J.  M.  WESTGATE,  Agronomist  in  Charge, 


Honolulu,  Hawaii. 

BULLETIN  No.  45. 


Under  the  supervision  of  the  STATES  RELATIONS  SERVICE, 
Office  of  Experiment  Stations,  U.  S.  Department  of  Agriculture. 


POTATO  DISEASES  IN  HAWAII 
AND  THEIR  CONTROL. 


BY 


C.  W.  CARPENTER,  Plant  Pathologist, 


Issued  January  24,  1920. 


DaSM&Aftr 


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U.S.  DEPOSITORY 


..^        WASHINGTON: 
GOVERNMENT  PRINTING  OFFICE. 

1920. 


HAW  An  AGRICULTURAL  EXPERIMENT  STATION, 
J.  M.  WESTGATE,  Agronomist  in  Charge, 

Honolulu,  Hawaii. 

BULLETIN  No.  45. 


Under  the  supervision  of  the  STATES  RELATIONS  SERVICE, 
Office  of  Experiment  Stations,  U.  S.  Department  of  Agriculture. 


POTATO  DISEASES  IN  HAWAII 
AND  THEIR  CONTROL. 

BY 

C.  W.  CARPENTER,  Plant  Pathologist. 


Issued  January  24,  1920. 


I 


W 


WASHINGTON: 
GOVERNMENT   PRINTING  OFFICE. 

1920. 


HAWAII  AGRICULTURAL  EXPERIMENT  STATION,  HONOLULU. 

[Under  the  supervision  of  A.  C.  True,  Director  of  the  States  Relations  Service.   United 
States  Department  of  Agriculture.] 

E.  W.  Allen,  Chief  of  Office  of  Experiment  Stations. 
Walter  H.  Evans,  Chief  of  Division  of  Insular  Stations,  Office  of  Experiment 

Stations. 

STATION  STAFF. 

J.  M.  Westgate,  Agronomist  in  Charge. 

J.  Edgar  Higgins,  Horticulturist. 

F.  G.  Krauss,  Superintendent  of  Extension  Work. 

C.  W.  Carpenter,  Plant  Pathologist. 

Wallace  Macfarlane,  Specialist  in  Soil  Fertility 

Investigations. 
K.  A.  Ching,  Assistant  Chemist. 
H.  L.  Chung,  Assistant  in  Agronomy. 
R.  A.  Goff,  In  Charge  of  Glenioood  Substation. 


LETTER  OF  TRANSMITTAL. 


Hawaii  Agricultural  Experiment  Station, 

Honolulu,  Hawaii,  October  25,  1918. 
Sir:  I  have  the  honor  to  submit  herewith  and  to  recommend  for  publication 
as  Bulletin  No.  45  of  the  Hawaii  Agricultural  Experiment  Station  a  paper  en- 
titled "Potato  Diseases  in  Hawaii  and  Their  Control,"  by  C.  W.  Carpenter, 
pathologist  of  the  station.  The  potential  as  well  as  actual  importance  of  the 
potato  industry  in  the  Hawaiian  Islands  makes  the  present  paper  both  timely 
and  valuable.  The  absence  of  cold  winter  weather,  together  with  the  almost 
continuous  cultivation  of  patches  to  potatoes  throughout  the  year  in  most  sec- 
tions, makes  the  problem  of  control  of  potato  diseases  much  more  difficult  than 
in  the  mainland  of  the  United  States. 
Respectfully, 

J.  M.  Westgate. 
Agronomist  in  Charge. 
Dr.  A.  C.  True, 

Director  States  Relations  Service, 

U.  S.  Department  of  Agriculture,  Washington,  D.  C. 

Publication  recommended. 
A.  C.  True,  Director. 

Publication  authorized. 
D.  F.  Houston, 

Secretary  of  Agriculture, 
2 


POTATO  DISEASES  IN  HAWAII  AND  THEIR 

CONTROL. 


CONTENTS. 


Page. 

Introduction 3 

Prevention    and    control    of    potato 

pests 4 

Potato   diseases    known    to    occur    in 

Hawaii 15 


Page. 
Potato  diseases  not  known  to  occur 

in    Hawaii 36 

Conclusion 41 


INTRODUCTION. 

The  growing  of  Irish  or  white  potatoes  in  Hawaii  was  at  one  time 
a  relatively  important  industry.  According  to  Sedgwick,1  in  the 
middle  of  the  last  century  potatoes  stood  at  the  head  of  the  list  of 
exports  and  in  1849  the  number  of  barrels  exported  was  51,957. 
Since  that  time  production  has  dwindled  and  consumption  increased 
until,  in  the  year  1916,  189,212  bushels  were  imported  from  the  coast 
States,  an  amount  approximately  equal  to  former  exports.  Various 
factors,  not  the  least  of  which  is  the  introduction  of  diseases,  have 
tended  to  make  the  growing  of  this  crop  difficult,  while  repeated  crop 
failures  and  discouraging  marketing  features  are  potent  elements  in 
lessening  production.  The  largely  increased  consumption  has  re- 
sulted from  the  proportionate  increase  in  the  number  of  people  using 
this  crop. 

A  considerable  quantity  of  potatoes  is  still  raised  in  the  islands  in 
spite  of  the  frequent  crop  failures.  The  greater  part  is  grown  in  the 
Kula  and  Makawao  sections  of  Maui  and  near  TVaimea  and  Hama- 
kua,  Hawaii.  Small  patches  of  from  one-quarter  to  5  acres  are  the 
rule,  and  a  considerable  quantity  is  grown  for  home  use.  An  ap- 
proximation as  to  the  present  annual  production  is  hard  to  reach, 
though  it  is  probably  not  in  excess  of  30,000  bushels. 

As  a  sound  agricultural  policy  it  is  desirable  that  this  languishing 
industry  be  fostered,  and  that  so  far  as  possible  the  Territory  be 
self-supporting  with  respect  to  this  commodity.  It  is  believed  that 
by  the  application  of  improved  cultural  practices,  fertilization,  and 
pest-control  methods  present  production  can  be  doubled  or  trebled 
without  any  increase  in  acreage.  The  present  rate  of  import  of 
this  staple  furnishes  an  indication  of  the  available  market. 

Since  the  writer  came  to  this  station  in  1916,  attention  has  been 
largely  devoted  to  a  study  of  the  potato  crop  and  the  reasons  for 

1  Sedgwick.  T.  F.     Potatoes.     Hawaii  Sta.  Rpt.  1901,  p.  374. 


4  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

crop  failures.  It  is  estimated  that  from  one  cause  or  another  at 
least  50  per  cent  of  the  crop  has  been  lost  during  the  period  under 
observation.  The  experience  of  the  past  two  years  justifies  the  con- 
clusion that  potato-crop  failures  in  the  islands  result  from  the  fol- 
lowing causes:  (1)  Use  of  poor  seed,  (2)  continuous  cropping,  (3) 
diseases  and  insect  pests,  and  (4)  unsatisfactory  soil  conditions. 
Fundamentally  diseases  and  insect  pests  are  the  most  important 
factors,  the  others  mentioned  being  merely  contributing  causes. 

The  diseases  which  have  come  under  observation  have  been  identi- 
fied, and  the  experience  gained  furnishes  ground  for  the  assumption 
that  control  measures  found  useful  for  combating  these  same  dis- 
eases in  other  sections  are  applicable  here  with  but  little  modifica- 
tion. It  is  realized  that  attention  has  not  been  devoted  to  this 
problem  for  a  sufficiently  long  period  to  furnish  a  basis  for  more 
than  tentative  conclusions  upon  many  of  the  problems  of  potato 
production,  but  this  bulletin  has  been  written  in  order  to  put  the 
results  of  the  study  into  the  nature  of  potato  failures  and  control 
measures  into  shape  suitable  for  meeting  the  increasing  demand  for 
information. 

Attention  is  directed  to  certain  cultural  practices  in  their  relation 
to  disease,  and  the  preventive  and  control  measures  which  have  been 
found  applicable  are  described.  Where  no  data  are  available  for 
Hawaiian  conditions,  information  as  to  methods  in  use  in  other 
localities  is  freely  drawn  upon.  The  diseases,  insect  pests,  and  other 
causes  of  lessened  yields  thus  far  observed  are  discussed.  Several 
serious  diseases  affecting  the  Irish  potato  in  other  localities  but  not 
yet  found  in  the  islands  are  described  in  order  that  Hawaiian  potato 
growers  may  guard  against  their  introduction. 

PREVENTION  AND  CONTROL  OF  POTATO  PESTS. 

GOOD   SEED. 

The  question  of  wThat  constitutes  good  seed  is  fundamental  in  the 
growing  of  a  healthy  potato  crop.  Good  seed  potatoes  should  be 
true  to  variety,  from  productive  plants,  firm,  free  from  disease, 
uniform  in  size  and  shape,  not  overripe,  and  not  weakened  by  sprout- 
ing. Such  seed  is  difficult  to  obtain  in  Hawaii.  It  is  probable  that 
satisfactory  local  seed  can  best  be  obtained  by  each  grower's  main- 
taining a  seed  plat,  or  possibly  a  few  growers  in  each  section  might 
well  specialize  in  seed  production. 

Certified  seed. — The  best  seed  is  that  from  the  mainland,  certified 
by  the  authorities  of  the  States.  The  label  "  Certified  Seed  "  shows 
that  the  seed  has  been  grown  under  a  system  of  State  inspection. 
According  to  the  varying  requirements  of  the  different  States  such 
seed  is  free  from  certain  diseases  and  shows  only  a  definite  allowable 
minimum  of  other  specified  diseases. 


POTATO   DISEASES   IX    HAWAII.  O 

The  mere  fact  that  the  seed  purchased  is  from  the  coast  is  not 
sufficient  indication  of  freedom  from  disease.  Imported  seed  and 
table  stock  used  for  seed  purposes  in  the  Territory  are  constantly 
found  infected  with  several  serious  diseases,  among  which  are 
Fusarium  wilt,  Rhizoctonia,  black  scurf,  corky  scab,  etc.  (See  figs. 
1  and  2.)  Unless  the  seed  is  carefully  inspected  there  is  always  the 
possibility  of  introducing  diseases  not  hitherto  prevalent  in  the 
islands.     If  mainland  seed  is  used,  the  certified  sort  should  be  in- 


Fig.  1. — Potato  diseases  not  controllable  by  treatment.  Sort  out  and  reject  all  tubers 
which  look  like  these  :  A,  Common  scab,  a  severe  case  ;  B,  internal  browning ;  C,  late 
blight  dry  rot ;  D,  Fusarium  dry  rot :  E,  blackleg  tuber  rot ;  F,  internal  browning  ;  G,  a 
wilt-infected  tuber  with  discolored  ring  at  the  stem  end.  (From  U.  S.  Dept.  Agr.,  Bur. 
Plant  Indus.,  C,  T..  and  F.  C.  D.  Circ.  3.) 

sisted  upon.  Before  planting  it  should  be  examined,  sorted,  if  neces- 
sary, and  disinfected  with  one  of  the  fungicidal  solutions  (p.  9). 

The  Fusarium  wilt  is  a  most  serious  disease  almost  constantly 
carried  by  imported  seed  (PI.  I)  and  very  prevalent  in  California. 
Probably  the  best  source  of  seed  for  Hawaii  is  the  Willamette 
Valley.  Oreg.,  where  the  Fusarium  wilt  is  of  infrequent  occurrence. 
The  seed  certification  boards  of  some  States  allow  a  small  percentage 
of  blackleg  in  stands  passed  as  certified  stock.  It  is  necessary,  there- 
fore, to  inspect  carefully  all  imported  seed  for  blackleg-infected 
tubers  (see  fig.  7,  p.  38)  and  to  sort  these  out  and  destroy  them. 

The  seed  plat. — In  the  chief  potato  sections  a  portion  of  the  farm 
should  be  set  aside  for  growing  seed  potatoes  for  subsequent  plant- 


6  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

ings.    Here  the  best  available  seed  should  be  planted  and  plants  care- 
fully watched  and  cared  for  through  the  growing  period. 

Roguing. — Koguing  the  seed  plat  is  practiced  with  the  object  of 
eliminating  undesirable  plants  before  the  tubers  are  harvested  in 
order  that  such  plants  may  not  be  propagated.  The  plants  are  ex- 
amined from  time  to  time  and  those  not  true  to  the  variety  or  those 
diseased  or  otherwise  undesirable  are  destroyed.  Varietal  mixtures 
can  best  be  detected  at  flowering  time.  In  roguing,  a  knowledge  of 
potato  diseases  is  valuable. 


Fig.  2. — Potatoes  that  may  be  planted  after  treatment :  A,  Black  scurf ;  B,  common  scab. 
Such  potatoes  may  be  planted  after  treatment,  but  it  is  better  still  to  select  perfect 
types  of  seed  tubers,  like  those  illustrated  by  C  and  D,  for  planting.  (From  U.  S.  Dept. 
Agr.,  Bur.  Plant  Indus.,  C,  T.,  and  F.  C.  D.  Circ.  3.) 

Seed  from  productive  plants. — As  many  unproductive  plants  are 
found  within  a  variety,  selection  should  be  practiced  within  the 
variety.  Stuart x  has  recorded  the  following  results  with  12  varieties, 
showing  the  average  yields  from  planting  the  progeny  of  strong  and 
weak  plants : 

Strong  tuber  units:  3.28  pounds  of  primes,  1.18  pounds  of  culls, 
total  4.46  pounds. 

Weak  tuber  units:  0.20  pound  of  primes,  0.51  pound  of  culls,  total 
0.71  pound. 

1  Stuart,  W.  Good  seed  potatoes  and  how  to  produce  them.  U.  S.  Dept.  Agr.,  Farmers' 
Bui.  533"  (1913),  p.  7. 


POTATO    DISEASES    IN    HAWAII.  7 

The  strong  plants  gave  more  than  10  times  as  large  a  yield  of 
primes  or  merchantable  tubers  and  only  a  little  more  than  twice  as 
many  culls  as  did  the  weak  plants. 

Uniformity. — Selection  has  shown  that  within  a  variety  there  are 
strains  which,  when  isolated,  behave  in  a  more  or  less  uniform  manner 
with  respect  to  size  and  shape  of  tubers.  Selection  should  be  prac- 
ticed from  the  hills  at  the  time  of  digging  the  seed  plat,  keeping  as 
specially  desirable  seed,  tubers  from  hills  yielding  a  maximum  num- 
ber of  healthy  tubers  of  fair  size  and  uniform  shape. 

Hill  selection. — This  method  of  selecting  seed  consists  in  marking, 
by  stake  or  otherwise,  the  most  desirable  plants  during  the  growing 
season.  At  digging  time  those  marked  plants  having  a  maximum 
number  of  desirable  tubers  are  specially  set  aside  for  seed.  In  this 
way  desirable  varietal  characters  of  the  plants  and  high-yielding 
qualities  are  selected  together. 

Immature  seed. — European  growers  have  come  to  believe  that,  other 
things  being  equal,  larger  crops  are  produced  by  immature  seed  than 
by  mature  seed.  This  refers  to  maturing  of  the  seed  in  the  ground. 
Most  of  the  locally  grown  seed  at  present  is  immature,  since  the  fields 
are  regularly  visited  by  blight  or  the  tops  dry  up  with  the  Fusarium 
wilt  disease,  etc.,  at  about  the  time  of  flowering. 

Large  v.  small  seed  tubers. — The  use  of  small  seed  tubers  can  be 
countenanced  only  when  these  are  known  to  be  the  progeny  of 
productive  plants.  From  the  quotation  from  Stuart  (p.  6)  regarding 
the  yield  of  strong  and  weak  plants  it  will  be  seen  that  the  strong 
plants  produced  more  than  16  times  as  great  a  weight  of  large  tubers 
as  the  weak  plants,  but  only  a  little  more  than  twice  as  great  a  weight 
of  small  tubers.  In  selecting  small  tubers  from  the  lot  it  is  evident 
that  a  large  proportion  of  tubers  from  unproductive  plants  would 
be  chosen. 

Ballou x  writes  as  follows  regarding  the  use  of  large  and  small 
tubers  for  seed : 

[The  use  of  large  tubers  gives]  :  (a)  A  very  heavy,  perhaps  almost  total, 
percentage  of  the  high-yielding  strains;  (b)  a  heavy  percentage  of  the  average 
or  moderate-yielding  strains;  (c)  a  very  small  percentage  of  the  inferior  or 
low-yielding  strains. 

[The  use  of  small  tubers  gives]  :  (a)  A  very  insignificant  percentage  of 
the  superior  or  high-yielding  strains;  (b)  a  small  percentage  of  the  moderate- 
yielding  strains;  (c)  a  very  heavy,  almost  total,  percentage  of  the  low-yielding 
or  inferior  strains. 

The  significance  of  the  above  data  with  respect  to  the  common  local 
practice  of  marketing  all  fair-sized  tubers  and  keeping  only  the  culls 
for  seed  needs  no  comment. 

1  Ballou,  F.  II.  The  status  of  the  potato-growing  industry  in  Ohio.  Ohio  Sta.  Bui.  218 
(1910),  p.  587. 


8  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

Whole  v.  cut  seed. — Provided  the  young  plant  is  furnished  with  a 
sufficient  supply  of  nourishment  by  the  seed  piece  until  able  to  elab- 
orate its  own  food,  there  does  not  appear  to  be  any  difference  whether 
the  seed  be  entire  tubers  or  cut  pieces.  It  is  true,  however,  that  within 
limits  the  larger  the  seed  piece  the  larger  will  be  the  resultant  crop, 
but  the  smaller  the  percentage  of  marketable  tubers.  Whole  tubers 
insure  a  good  stand  with  greater  freedom  from  disease.  However, 
the  growth  of  too  many  sprouts  is  to  be  discouraged,  since  too  great 
a  number  of  tubers  will  be  set  and  comparatively  few  will  be  able  to 
reach  a  satisfactory  size. 

Greening  the  seed  by  exposure  to  light. — In  order  to  hasten  the 
germination  or  sprouting  of  seed,  that  is,  to  shorten  the  rest  period, 
it  is  a  common  practice  in  Kula,  Maui,  to  spread  the  tubers  to  be 
used  as  seed,  as  soon  as  they  are  dug,  under  a  thin  shade  of  bushes 
for  several  weeks  or  until  they  sprout.  It  is  claimed  that  they  will 
sprout  in  about  half  the  ordinary  dormant  period.  There  is  as  yet 
no  experimental  evidence  to  support  or  controvert  this  statement  of 
the  farmers.  This  exposure,  especialty  to  the  high  temperature  of 
midday,  probably  serves  to  prevent  any  tendency  of  the  seed  to  rot 
from  the  late  blight  fungus,  but  results  in  loss  from  other  rots  and 
from  such  insects  as  the  tuber  moth.  The  shortened  period  of  dor- 
mancy, if  a  fact,  is  advantageous  when  potatoes  are  grown  almost 
continuously  and  seed  is  difficult  to  obtain  at  certain  seasons. 

Seed  selection  in  relation  to  disease  control. — Seed  selection  is  not 
only  of  value  in  obtaining  pure  high-yielding  strains  within  a  va- 
riety, but  it  is  of  fundamental  importance  in  preventing  those  diseases 
which  are  carried  by  the  seed.  Such  supposedly  nonparasitic  but 
inherent  diseases  as  curly  dwarf,  leaf  roll,  etc. ;  the  fungus  diseases 
late  blight,  Fusarium  wilt,  rosette  (Rhizoctonia),  and  those  due  to 
the  scab  fungi ;  as  well  as  certain  insect  pests,  are  carried  by  the  seed. 
For  avoiding  these  diseases  any  of  the  following  well-known  methods 
of  selection  are  available:  (1)  Selection  from  the  pile  or  lot,  (2)  field 
selection, (3)  hill  selection,  and  (4)  regional  selection  and  use  of  cer- 
tified seed. 

Selection  of  healthy  tubers  of  uniform  size  and  shape  for  seed  from 
the  mass  is  valuable  in  disease  control,  though  not  necessarily  so 
in  obtaining  productive  strains.  As  mentioned  above,  hill  selection 
is  of  value  both  in  disease  control  and  in  obtaining  productive  strains. 
Regional  selection  and  the  use  of  certified  seed  may  well  serve  as 
means  of  starting  seed  plats  in  Hawaii.  Seed  should  not  be  obtained 
from  regions  or  from  fields  where  noxious  pests  are  known  to  be 
prevalent. 

Cutting  seed. — In  cutting  potatoes  for  seed  the  tubers  must  be  so 
cut  that  each  seed  piece  or  set  shall  have  at  least  one  strong  eye.  and 
it  is  desirable  that  each  shall  weigh  about  1J  ounces.  First  cut  a 
thin  slice  from  the  stem  end  of  the  tuber  and  examine  the  flesh  for 


POTATO   DISEASES   IX    HAWAII. 


discoloration  of  the  vascular  or  woody  ring.  Brownish  or  black 
discoloration  at  this  point  indicates  Fusarium  wilt  or  other  infection. 
Such  tubers  are  not  suitable  for  seed.  Then  cut  the  tuber  length- 
wise through  the  bud  cluster  at  the  end  and  through  to  the  stem  end. 
Then  divide  each  half  crosswise.  With  large  tubers  further  division 
can  be  made,  but  the  cuts  should  always  be  so  made  as  to  insure 
blocky  pieces  when  possible.  It  requires  about  22  bushels  of  seed 
adequately  to  plant  an  acre  at  the  standard  distance  of  rows  3  feet 
apart  and  seed  pieces  1  foot  apart  in  the  row.  Less  than  half  this 
amount  is  commonly  planted. 

Potato  seed  disinfection. — Diseases  such  as  corky  scab,  Rhizoctonia 
scab,  and  rosette  (PI.  VIII),  which  are  carried  on  the  surface  of 
the  seed,  can  be  controlled  in  some  degree  by  soaking  the  seed  in 
a  disinfecting  solution.  Seed  disinfection  will  be  of  little  value  if 
the  soil  is  already  infected  with  the  disease  for  which  the  seed  is 
treated.  It  is  on  the  whole  a  better  practice  not  to  cut  the  tubers 
before  soaking  them  in  the  disinfecting  solution.  For  immediate 
planting,  however,  the  tubers  may  be  cut  before  dipping. 

The  solutions  most  frequently  employed  for  potato  disinfection 
are  formalin  and  corrosive  sublimate.  Either  is  suitable  for  the 
cork}7  scab,  but  the  evidence  is  in  favor  of  the  latter  solution  for 
the  Ehizoctonia  diseases  (black  scurf  and  rosette). 

The  usual  formalin  solution  is  made  up  as  follows : 

Formalin    (40  per   cent   formaldehyde) pint 1 

Water   to   make gallons—  30 

The  sacks  containing  the  potatoes  should  be  immersed  in  this 
solution  for  two  hours,  after  which  the  tubers  should  be  removed 
and  spread  to  dry.  Thirty  gallons  of  the  solution  is  sufficient  for 
disinfecting  about  30  bushels  of  seed. 

Corrosive  sublimate  has  the  disadvantage  of  being  a  deadly  inter- 
nal poison,  and  it  should  be  handled  with  this  fact  always  in  mind. 
The  solution  corrodes  metal  and  therefore  only  wood  and  nonmetal- 
lic  containers  should  be  used.     The  formula  is  as  follows : 

Corrosive    sublimate ounces 4 

Water    gallons—  30 

Dissolve  the  chemical  in  a  few  gallons  of  hot  water  and  dilute  to 
30  gallons.  Immerse  the  sacks  containing  the  tubers  for  two  hours. 
then  remove  and  spread  the  potatoes  to  dry.  As  this  solution  loses 
its  strength  after  treating  four  to  six  lots  of  potatoes,  fresh  solution 
should  be  substituted. 

SUITABLE    POTATO   SOIL. 

Kind  of  soil  and  method  of  treatment. — So  far  as  possible  soil 
free  from  potato  diseases  should  be  chosen.  As  the  Irish  potato  is 
a  cool-climate  crop,  in  the  Hawaiian  Islands  it  is  at  home  at  an  ele- 


10  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

vation  of  2,000  to  3.000  feet  or  more  on  rather  loose-textured,  well- 
drained  soils.  In  any  case  heavy  wet  soils  poorly  drained  and  hard 
to  work  are  not  suitable  for  potato  culture.  In  some  potato  soils  in 
Kula,  Maui,  the  presence  of  a  layer  of  hardpan  a  few  inches  under 
the  surface  indicates  that  shallow  surface  plowing  has  too  long  been 
practiced  and  that  the  conditions  of  apparent  soil  exhaustion  might 
be  largely  improved  in  such  fields  by  deeper  plowing.  Hilling  up 
the  plants  might  well  receive  more  attention  as  an  aid  in  checking 
attack  by  the  potato  tuber  moth.  Crop  rotation  should  be  prac- 
ticed more  and  more  here,  not  only  to  increase  the  fertility  of  the 
soil  but  also  to  combat  the  prevalent  diseases  and  insect  pests.  Seed 
selection  is  almost  useless  or  is  of  temporary  value  unless  combined 
with  rotation.  The  organisms  causing  Fusarium  wilt,  rosette,  black 
scurf,  corky  scab,  etc.,  live  indefinitely  in  the  soil,  but  it  is  generally 
believed  that  in  the  absence  of  susceptible  plants  the  virility  of  the 
organism  is  lessened  and  the  number  decreased.  Similarly,  the 
tuber  moth,  which  is  most  serious  in  dry  years,  can  possibly  be  held 
under  control  by  clean  cultivation  and  rotation  combined  with  in- 
telligent spraying  with  arsenicals.  Crop  rotation,  combined  with 
the  use  of  good  seed  and  preventive  sprays,  offers  promising  oppor- 
tunities of  improving  the  Hawaiian  potato  industry  through  in- 
creased yields  and  better  quality. 

Soil  reaction. — As  certain  organisms  are  favored  by  an  acid  soil 
while  others  are  encouraged  by  an  alkaline  soil,  the  latter  being 
favorable  to  the  development  of  corky  scab,  lime  or  wood  ashes 
should  never  be  applied  to  potato  fields,  as  they  tend  to  produce  an 
alkaline  soil  reaction  and  are  sure  to  encourage  scab.  On  the  other 
hand,  the  plowing  under  of  green-manure  crops  will  tend  to  make 
the  soil  acid  and  thus  reduce  the  damage  from  this  disease.  The 
Ehizoctonia  diseases  are  thought  to  be  worse  on  heavy,  poorly 
drained,  acid  soils.  Correcting  the  drainage  and  aeration  of  such 
soils  is  beneficial. 

SPRAYING,   SPRAY   MIXTURES,   BAITS,  ETC. 

In  order  to  protect  the  potato  plant  against  various  insect  pests  and 
fungus  diseases,  sundry  chemicals  are  sprayed  or  dusted  thereon  or 
used  as  baits.  The  chemical  used  varies  with  the  nature  of  the  pest — 
there  are  no  cure-alls.  The  material  may  serve  as  a  protection  of  the 
foliage  against  fungus  infection  or  as  a  contact  or  internal  poison  for 
insect  pests.  In  certain  weather  conditions,  a  coating  of  spray  may 
act  mechanically  to  prevent  sun  injury  of  the  foliage. 

Bordeaux  mixture  for  the  control  of  foliage  diseases. — The  most 
widely  used  fungicidal  spray  for  the  prevention  of  foliage  diseases  of 
the  potato  is  Bordeaux  mixture.  The  experiments  carried  on  with 
this  spray  by  the  writer  alone  and  cooperatively  with  others  in 


POTATO    DISEASES    IX    HAWAII.  11 

Hawaii  for  the  prevention  of  the  late  blight  disease  have  been  very 
encouraging.  The  fact  that  late  blight  is  present  upon  almost  every 
potato  crop  grown,  and  that  when  it  is  too  dry  for  this  disease  the 
early  blight  is  serious,  should  be  sufficient  argument  in  favor  of  uni- 
versal spraying  of  potatoes  with  Bordeaux. 

Standard  Bordeaux  mixture  is  made  up  according  to  the  following 
formula,  which  is  often  referred  to  as  1 : 1 :  10  or  5 :  5 :  50  Bordeaux. 

Bluestone   (copper  sulphate) pound—     1 

Quicklime    (not  air  slaked) do —     1 

Water  to  make gallons__  10 

The  mixture  is  prepared  by  dissolving  the  bluestone  by  hanging  it, 
preferably  overnight,  in  a  sack  immersed,  the  top  just  under  the  sur- 
face, in  a  gallon  of  water  in  a  wooden  container.  If  time  is  a  factor, 
pulverize  the  bluestone  and  dissolve  it  in  a  gallon  of  hot  water. 
Dilute  to  5  gallons  Make  a  lime  paste  by  slaking  the  pound  of  lime 
in  a  small  quantity  of  water,  adding  enough  water  to  prevent  its  boil- 
ing dry.  When  the  boiling  ceases,  stir  to  a  smooth  cream  and  add 
water  to  make  5  gallons.  Just  before  the  Bordeaux  mixture  is  re- 
quired for  use,  pour  the  5  gallons  of  diluted  bluestone  and  5  gallons 
of  diluted  lime  at  the  same  time  into  a  wooden  container  and  stir 
vigorously.  Stir  and  strain  into  the  spray  tank  and  agitate  occa- 
sionally while  spraying,  as  even  properly  made  Bordeaux  settles 
gradually.  The  bluestone  solution  and  the  lime  solution  can  be  kept, 
but  Bordeaux  mixture  should  be  used  the  day  it  is  made.  However, 
if  sugar  is  stirred  in  at  the  rate  of  ^  pound  to  10  gallons  the  day 
the  mixture  is  made,  Bordeaux  may  be  kept  several  days  in  covered 
wood  containers. 

Where  considerable  spraying  is  to  be  done,  it  is  advantageous  to 
prepare  stock  solutions  of  the  bluestone  and  of  the  lime.  It  is  con- 
venient to  prepare  these  solutions  so  that  each  gallon  contains  a 
pound  of  chemical.  Then  to  make  50  gallons  of  Bordeaux,  it  is  neces- 
sary only  to  take  5  gallons  of  the  bluestone  stock  solution  and  dilute 
it  to  25  gallons,  to  dilute  similarly  5  gallons  of  the  lime  stock  solu- 
tion, and  to  mix  the  two  diluted  solutions. 

It  is  as  easy  to  prepare  Bordeaux  mixture  by  the  right  method  as 
by  any  other,  and  properly  made  Bordeaux  is  much  more  effective 
than  the  mixtures  of  uncertain  physical  and  chemical  composition 
which  result  from  haphazard  methods.  For  the  most  effective  Bor- 
deaux it  i>  essential  that  the  bluestone  solution  and  the  lime  suspen- 
sion be  diluted  before  combining.  When  properly  made  and  properly 
applied,  Bordeaux  has  remarkable  adhesive  properties,  once  it  be- 
comes dry  on  foliage. 

The  active  principle  of  Bordeaux  mixture  is  the  copper,  but  in 
order  not  to  injure  the  foliage  and  to  render  the  treatment  more  last- 


12 


BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 


ing  the  copper  is  made  insoluble  by  the  use  of  lime.  When  the  di- 
luted solutions  come  together  minute  precipitation  membranes  are 
formed.  In  spraying,  the  plant  is  covered  with  a  thin  layer  of  these 
minute  membranes.  The  copper  in  the  membranes,  rendered  slowly 
soluble  by  the  action  of  the  carbon  dioxid  of  the  air,  forms  a  solu- 
tion in  any  minute  films  and  droplets  of  water  on  the  leaves,  which 
prevents  the  germination  of  fungus  spores. 

The  Bordeaux  should  be  applied  on  both  upper  and  lower  sur- 
faces of  the  leaves  and  at  the  highest  pressure  obtainable  to  insure 
thorough  protection  and  economic  distribution.  Each  plant  should 
be  sprayed  long  enough  to  cover  all  parts  but  not  sufficiently  long 

to  allow  the  spray  to 
collect  in  drops  and 
run  off,  as  in  the  latter 
case  the  spray  is  not 
only  wasted  but  is  less 
adhesive.  The  plants 
will  hardly  show  the 
spray  when  it  has 
been  properly  and 
thoroughly  applied 
with  sufficient  pres- 
sure and  in  the  desir- 
able misty  form.  ( See 
fig.  3.) 

Perhaps  under  Ha- 
waiian conditions 
some  variations  of  the 
formula  for  Bordeaux 

Fig.  3. — Desirable  types  of  hand  sprayers,  with  extension  as  given  may  be  found 
rods  permitting  thorough  underspraying  of  the  foliage.       advisable        Possiblv  4- 

pounds  of  Milestone  and  4  pounds  of  lime  in  50  gallons  of  the  mix- 
ture would  do  the  work  satisfactorily. 

Where  there  is  difficulty  in  spraying  plants  due  to  the  waxy  sur- 
face of  the  leaves,  or  where  the  spray  does  not  stay  on  owing  to  heavy 
rains,  a  sticker  made  up  according  to  the  following  formula  may  be 
used  to  advantage : 

Resin  pound--  1 

Sal  soda   (crystals) do y2 

Water    gallon y2 

The  ingredients  should  be  boiled  together  until  a  clear  brown 
sirupy  liquid  appears,  then  cooled  and  added  to  50  gallons  of 
Bordeaux  mixture  (or  proportionately  for  smaller  quantities  of 
spray). 


POTATO    DISEASES    IN    HAWAII.  13 

Modification*  of  Bordeaux  mUturc  for  the  control  of  leaf -eating 
insects. — For  controlling  leaf-eating  insects,  such  as  the  army  worm 
or  "  poko,"  and  as  a  possible  aid  in  combating  the  tuber  moth.  Paris 
green  may  be  used  in  Bordeaux  at  the  rate  of  1  pound  to  50  gallons. 
Usually  Bordeaux  mixture  contains  a  sufficient  excess  of  lime  to 
neutralize  the  Paris  green  and  no  burning  of  the  foliage  results. 
but  if  lime  of  doubtful  quality  is  used  in  preparing  the  Bordeaux. 
an  amount  of  lime  equal  in  weight  to  the  Paris  green  should  be  added 
to  prevent  burning. 

The  usual  method  of  destroying  leaf-cutting  insects  (fig.  5.  p.  31), 
however,  is  by  adding  arsenate  of  lead  to  Bordeaux  mixture  at  the  rate 
of  about  3  pounds  to  50  gallons.  Whether  this  form  of  poisoning 
will  prove  more  satisfactory  for  the  leaf-eating  cutworms  or  "  pokos  " 
than  the  locally  commonly  used  flour-Paris-green-dust  bait  remains 
to  be  seen.  For  the  tuber  moth,  arsenate  of  lead  in  Bordeaux  will 
probably  be  of  considerable  benefit. 

Lime-sulphur  spray  or  dry  sulphur  for  the  control  of  mites. — For 
the  control  of  the  potato  mite  (Pis.  XII  and  XIII).  which  in  dry 
and  hot  situations  causes  the  death  of  young  growth  and  premature 
development  of  the  plant,  a  lime-sulphur  spray  made  up  as  follows 
has  been  found  effective : 

Sulphur    pound 1 

Quicklime    do 1 

Water  to  make gallons 20 

Boil  the  sulphur  and  quicklime  in  a  gallon  of  water  in  a  kettle 
or  pan  until  they  combine  into  a  yellowish  sirupy  liquid,  this  usually 
requiring  about  three-quarters  of  an  hour.  Dilute  to  make  20  gal- 
lons of  spray  mixture. 

Dry  surphur  dusted  upon  the  foliage  with  an  insect-powder  blower 
is  likewise  effective  in  the  control  of  mites. 

Soil  fungicides  far  Sclerotium  wilt. — Ammoniacal  copper  carbon- 
ate solution  and  "eau  celeste."  which  contain  copper  in  soluble  form. 
are  recommended  as  soil  fungicides  in  case  of  Sclerotium  wilt 
(p.  26).  The  chemicals  needed  to  make  sufficient  solution  for  50 
gallons  of  copper  carbonate  fungicide  are  as  follows : 

Copper   carbonate ounces__       5 

Ammonia   <  2G°  Baume) pints__       3 

Water  to  make gallons—     50 

To  a  gallon  of  water  in  a  wooden  vessel  add  the  ammonia  and  stir. 
Add  the  copper  carbonate  a  little  at  a  time,  stirring  constantly.  Con- 
tinue to  add  the  chemical  until  no  more  will  dissolve.  Allow  any 
undissolved  carbonate  to  settle  to  the  bottom  and  draw  off  the  clear 
blue  supernatant  liquid.     This  solution  does  not  keep  well  for  more 


14  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

than  a  few  days,  and  is  best  prepared  fresh  as  needed.     For  use 
dilute  the  solution  to  50  gallons. 

Since  Milestone  may  be  more  readily  obtained  than  copper  car- 
bonate, the  formula  for  another  soil  fungicide,  eau  celeste,  is  given. 

Bluestone  (copper  sulphate) pounds..      2 

Ammonia pints..       3 

Water  to  make gallons 50 

Dissolve  the  bluestone  in  a  gallon  of  water.  When  dissolved,  add 
the  ammonia  to  it.  Transfer  to  bottles  that  can  be  tightly  corked  if 
the  solution  is  not  to  be  used  at  once.  For  use  dilute  to  50  gallons. 
A  half-teacupful  of  either  of  these  solutions  sprayed  in  time  at  the 
base  of  each  plant  is  said  to  protect  the  plant  against  Sclerotium  wilt. 

Poison  baits  for  army  worms. — Where  an  invasion  of  army  worms 
comes  from  adjacent  grassland,  cultivated  fields  may  be  protected 
by  surrounding  with  a  line  of  poison  bait  or  establishing  such  a  line 
along  the  threatened  side.  Ditches  sprinkled  with  lime  serve  the 
same  purpose. 

Paris-green  bran  mash  as  bait  for  cutworms. — With  25  pounds  of 
bran  thoroughly  mix  while  dry  \  pound  Paris  green.  To  make  the 
bait  attractive  chop  fine  six  lemons  or  waste  citrus  product,  papaya, 
etc..  and  add  to  the  mixture,  or  add  a  quart  of  cheap  molasses. 
Thoroughly  mix  and  add  sufficient  water  to  moisten  the  mixture,  but 
not  enough  to  make  it  sloppy  and  thus  interfere  with  its  easy  and 
economical  distribution. 

Arsenate  of  lead  may  be  substituted  for  the  Paris  green,  using 
four  to  six  times  as  much,  or  of  white  arsenic  half  as  much  as  of 
Paris  green  may  be  used. 

Criddle  mixture  for  cutworms. — The  following  formula  is  largely 
quoted  as  an  effective  bait  for  cutworms.  As  a  cheap  substitute  for 
cereal  baits  it  is  well  worth  trying. 

Fresh  horse  dung pounds..  60 

Salt do 2 

Paris  green pound..     1 

The  Paris  green  is  mixed  with  enough  water  to  form  a  thin  paste 
and  it  is  then  thoroughly  mixed  with  the  horse  dung. 

HANDLING,   SORTING,   GRADING,  AND   STORING. 

To  prevent  bruising,  potatoes  should  be  carefully  handled  at  all 
times.  The  slightest  wound  is  sufficient  to  open  the  way  for  the 
entrance  of  rot-producing  organisms.  Care  in  handling  is  especially 
necessary  at  digging  time  when  the  skins  are  tender. 

All  bruised  and  rotted  tubers  should  be  sorted  out  before  shipping 
or  storing,  and  if  the  crop  is  to  be  stored  any  length  of  time  re-sorting 


POTATO    DISEASES    IX    HAWAII.  15 

may  be  necessary.  Potatoes  carefully  graded  as  to  size  are  much 
more  readily  sold  at  a  good  price  than  when  large  and  small  are 
mixed.  All  potatoes  of  inferior  size  should  be  soiled  out  and  used 
on  the  farm.  It  is  a  poor  policy  to  leave  in  the  shipment  any  bruised 
or  decaying  tubers,  as  these  are  valueless  and  by  wetting  and  other- 
wise making  the  lot  unattractive  they  may  result  in  considerable  loss 
to  the  shipper  through  market  charges  for  re-sorting. 

Outside  the  cold-storage  warehouses  a  sufficiently  cold  place  for 
the  storage  of  potatoes  is  difficult  to  maintain  in  Hawaii.  For  the 
best  results  a  temperature  of  about  35°  F.  is  required.  It  is  probable 
that  houses  quite  suitable  for  the  storage  of  this  crop  could  be  con- 
structed at  the  higher  altitudes  where  potatoes  are  most  successfully 
grown  should  there  develop  sufficient  economic  advantage  in  holding 
the  crop.  Since  the  potato  consumption  in  the  islands  is  so  greatly  in 
excess  of  the  production,  there  is  little  incentive  to  store.  However,  a 
small  storage  place,  screened  against  insects,  tightly  built,  and  pro- 
vided with  close-fitting  doors,  so  as  to  take  advantage  of  the  night 
temperature,  would  be  valuable  in  preventing  loss  from  shipping  to 
a  glutted  market.  The  place  could  be  used  for  holding  seed  stock, 
for  fumigating  and  storing  beans,  corn.  etc. 

POTATO  DISEASES  KNOWN  TO  OCCUR  IN  HAWAII. 
CLASSIFICATION. 

In  order  that  plant  diseases  may  be  intelligently  combated,  an 
understanding  of  their  nature  is  necessary.  According  to  their 
causes  the  diseases  to  which  the  potato  is  subject  may  be  divided  into 
two  classes:  (1)  Those  caused  by  organisms,  as  fungi,  bacteria,  in- 
sects, etc..  which  may  be  called  parasitic  diseases:  (2)  those  induced 
by  unfavorable  growing  conditions  or  by  obscure  and  undetermined 
causes,  which  may  be  termed  nonparasitic  diseases. 

The  parasitic  organisms  which  cause  diseases  of  the  first  group 
may  be  present  in  the  soil,  introduced  on  the  seed,  or  brought  to  the 
soil  and  the  growing  crop  from  adjacent  fields  by  wind  or  water., 
Under  favoring  conditions  in  the  presence  of  the  potato  plant,  the 
disease  progresses  more  or  less  virulently.  The  following  parasitic 
diseases,  grouped  according  to  the  nature  of  the  parasite  as  fungus 
or  insect,  have  been  found  factors  in  potato  production  in  Hawaii: 
Fungus  diseases,  including  Fusarium  wilt  {Fusarium  oxysporum), 
late  blight  (Phytophthora  infestans).  black  scurf  and  rosette  {Rhiz- 
octonia  solan  i).  early  blight  (AUernaria  solani).  common  or  corky 
scab  {Actinomyces  chromo genus),  tuber  rots  {Fusarium  oxysponun. 
F.  radicicolO)  and  F.  coeruleum)  :  insect  pe>ts.  including  tuber  moth 
{Phthorimcea  operculeUu).  cutworms  and  a  similar  leaf-eating  worm 


1  6  BULLETIN   45,    HAWAII,  EXPERIMENT   STATION. 

locally  called   "  poko,"  and  mites    (unidentified   form   of  Tetrany- 
chida?). 

The  nonparasitic- diseases  of  the  potato  may  be  occasioned  by  un- 
favorable environment  such  as  unsatisfactory  soil  composition,  un- 
even growth  due  to  prevailing  weather  conditions,  or  mechanical  or 
chemical  injury  from  injudicious  applications  of  arsenical  sprays. 
Bright,  hot  sunlight  after  certain  kinds  of  weather  frequently  causes 
sunscald  and  tipburn  of  the  leaves.  It  may  be  noted  that  many  of 
the  diseases  usually  classed  as  nonparasitic,  though  imperfectly  un- 
derstood, can  be  controlled  through  seed  selection  and  elimination  of 
undesirable  strains.  Such  are  the  inherited  diseases,  leaf  roll,  curly 
dwarf,  and  mosaic,  which  probably  will  not  be  serious  factors  in  the 
Hawaiian  potato  industry. 

IDENTIFICATION. 

Many  times  a  knowledge  on  the  part  of  the  grower  of  the  different 
diseases  which  attack  the  potato  would  save  crops  now  needlessly 
lost  through  failure  at  the  critical  time  to  apply  the  proper  treatment 
to  hold  the  disease  in  check. 

Plants  which  appear  unhealthy  for  any  reason  should  be  examined 
with  the  following  points  in  mind;  and  with  the  accompanying  key 
as  a  guide,  a  working  knowledge  of  epidemic  diseases  in  Hawaii  is 
soon  acquired.  All  growers  should  learn  at  least  to  recognize  late 
blight,  early  blight,  mite  disease,  Fusarium  wilt,  and  rosette. 

Points  to  be  Noted  in  Examining  a  Plant. 

Note  how  the  plant  differs  from  the  normal.  If  the  leaves  have 
diseased  spots,  note  their  character.  If  the  leaves  wilt,  note  which 
ones  do  so,  the  new  small  leaves  or  the  lower  older  leaves.  The 
presence  of  worms  in  the  leaves  and  stems  indicates  tuber  moth. 
If  the  new  top  growth  curls,  twists,  becomes  fuzzy,  and  dries  up, 
while  at  the  same  time  the  leaves  become  bronzed  on  the  lower  side, 
it  is  the  work  of  the  potato  mite.  If  the  lower  leaves  wilt  and  drop 
off  more  than  normally  or  some'  leaves  roll  up  in  tubular  form  and 
no  diseased  spots  are  present,  examine  the  underground  stem  near 
the  soil  level.  If  it  is  girdled  with  a  cankerous,  dry,  brown  lesion, 
Ehizoctonia  may  be  present,  causing  rosette  disease,  or  if  the  stem 
is  here  a  shell  with  the  center  rotted  out,  the  plant  is  perhaps  at- 
tacked by  Sclerotium  wilt.  Sometimes  the  stem  borer  does  similar 
damage,  in  which  case  the  burrow  is  readily  traced  and  the  worm 
found.  If  the  stem  appears  normal  externally,  split  it  with  a 
knife  and  examine  the  inside  at  the  ground  level  and  below.  If 
the  vascular  tissue — that  is,  the  woody  portion  a  short  distance  in 
from  the  surface — shows  a 'brown  discoloration  which  is  traceable 
toward  the  root  system,  the  plant  is  suffering  from  lack  of  water 
because  its  water-conducting  apparatus  is  plugged  by  fungus  threads 
(Fusarium  wilt). 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


Plate  I. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


Plate  II. 


Potato  Leaves  Showing  Late  Blight. 


Bui.  45.  Hawaii  Agr.  Expt.  Station 


Fig.  I.— Rows  at  Left  Sprayed  Once  with  Bordeaux  Mixture, 
Rows  in  Center  not  Sprayed. 


Fig.  2—  Center  Rows  Sprayed,  Outside  Rows  not  Sprayed. 


Fig.  3.— Potatoes  Piled  for  Comparison. 
POTATO    SPRAYING    EXPERIMENTS. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


PLATE    IV. 


Fig.  I. —Seven-Acre  Field  of  Potatoes  in  which  Late  Blight  was  Controlled 
by  Thorough  Spraying  with  Bordeaux  Mixture. 


Fig.  2— Yield  of  10  Average  Hills  in  Above  Field;  Weight.  17*  Pounds. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


Plate  V. 


Fig.  I —Comparative  Test  of  Early  Rose  and  Hamakua 
Hybrid  Varieties-  Early  Rose  Dead  of  Late  Blight 
in  60  Days. 


Fig.  2— Yield  of  96  Hills  of  Early   Rose  Variety  at  60 
Days:  Weight,  71  Pounds. 


Fig.  3.— Yield  of  20  Hills  of  Hamakua  Hybrid  Variety  at 
90  Days,-  Weight,  9'  Pounds. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 

Plate  VI. 

A 

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r] 

f| 

'•  V-/ 

/ 

\ 

- 

/ 

; 

-v 

;      , 

• 

:/r^ 

1 

\ . 

Early  or  Dry  Blight  of  Potatoes. 

Note  concentric  rings  in  diseased  spots. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


Plate  VII. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


PLATE   VIII 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


PLATE    IX. 


Leaf  Roll  and  Rosette  Associated  with  Stem  Lesions  Due  to  Rhizoctonia. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


PLATE    X. 


Aerial  Tubers  Produced  as  a  Result  of  Stem  Girdling  by  Rhizoctonia. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 

Plate  XI. 

^^^**mP 

iH^  ^H 

j'**        1 

t^HHr 

n 

Sr                                 "   ^B 1        * 

*9*          •      *1$. 

-V    ^ 

J  j^»'          ^H 

n    > 

B                V 

iP*  -    *          ^ 

;              |tj3M 

■■5                  ^ 

■j 

_rJ 

i                    1 

j^Bl 

m.                                                          ^1    ' 

^^^P^2k       •fP7VA 

ifl 

V^SS^fc^*Ll  -  ">-?^^*|r^ 

^^^_2^D«^| 

Fig.  I. —Potato  Section  Showing  Injury  by  Larv/e  of  Tuber  Moth. 

(From  U.  S.  Dept.  Agr.  Bui.  427.) 


Fig.  2.— Injury  by  Potato  Tuber  Moth  to  Potato  Plants,  Showing 
Mines  in  Leaves,  Petioles,  and  Stems. 

(From  U.  S.  Dept.  Agr.  Bui.  427.) 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


PLATE   XII. 


Mite  Disease  of  Potato  Showing  Characteristic  Appearance  of  Buds,  Shoots, 
and  Leaves.    Adult  Mite,  Young  Mite,  and  Egg.     X   160, 


Bui.  45,  Hawaii  Agr.  Expt.  Station 


Plate  XIII, 


Fig.  I— Mite- Infested  Potato  Plant. 


Fig.  2.— Above  Plant  10  Days  after  Spraying  with  Lime-Sulphur. 


Fig.  3— Left,  Yield  of  50  Plants  Sprayed  Four  Times,  Weight  of  Tubers 
49  Pounds;  Right,  Yield  of  50  Unsprayed  Plants.  Weight  of  Tubers  24 
Pounds. 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


Plate  XIV. 


Fig.  I. —Nematode- Infested  Potatoes. 


Fig.  2.— Nematode-Infested  Tubers  Cut  to  Show  the  Watery  Areas  or  Spots 

Near  the  Peel. 


POTATO   DISEASES   IN    HAWAII.  17 

Key  to  Potato  Diseases. 

Foliage  diseases : 

Leaf  spots. 

Spots  small,  one-sixteenth  to  one-fourth  inch  in  diameter, 
rounded  to  angular,  brown,  and  becoming  dry  and  falling 
out,  leaving  shothole  effect.  Spots  often  marked  with  con- 
centric lines  or  successive  borders  of  growth.  Prevalent  in 
dry  seasons. 

Early  blight  (PI.  VI  and  p.  23). 
Spots  one-fourth  to  one-half  inch  or  more  in  diameter,  rounded, 
spreading  rapidly,  and  bordered  by  lighter  yellow-green 
zone ;  on  underside  of  leaf  in  damp  weather  a  delicate  frost- 
like mildew  may  be  seen ;  in  wet  weather  the  plants  entirely 
rot  in  a  few  days.  Preventive  treatment  must  be  given 
before  disease  starts  or  as  soon  thereafter  as  possible;  if 
plants  are  badly  attacked,  spraying  will  not  be  worth 
while. 

Late  blight  (PI.  II  and  p.  20 L 
Wilting  and  rolling  of  leaves,  etc. 

Young  leaves  and  new  growth  first  affected.  Leaves  bronzed 
on  underside,  twisted,  and  curled  up,  becoming  fuzzy  and 
drying  up,  the  plant  dying  from  the  top  downward.  Mites 
can  be  found  with  hand  lens.  Prevalent  in  the  dry  season. 
Potato  mite  disease  (Pis.  XII  and  XIII  and  p.  31). 
Plant  appears  to  suffer  from  lack  of  water  and  dies  prema- 
turely ;  lower  leaves  wilt  and  drop  off ;  interior  of  lower 
stem  at  ground  level  shows  browning  of  vascular  or  woody 
portion. 

Fusarium  wilt  (PI.  I  and  p.  18). 
Leaves  more  or  less  rolled  and  tubular ;  plant  spreads  out  and 
grows  all  to  top ;  tubers  few,  small,  and  set  close  to  stem 
in  a  bunch  or  forced  out  of  the  soil;  tubers  forming  in 
axils  or  leaves  in  advanced  cases;  stem  girdled  below 
ground  with  brown  cankerous  lesion. 

(Rosette  (Rhizoctonia)   (Pis.  VII,  VIII,  and  IX  and  p.  24). 
Lower  leaves  wilt ;  stem  at  soil  level  rotted  through  or  hollow, 
with    white    fungus   growth   producing    mustard-seed-like 
sclerotia. 

Sclerotium  wilt  (p.  25). 

Leaves  roll  up  in  tubular  form,  or  plant  wilts  and  generally 

appears  as  if  the  stem  were  broken  off;  interior  of  stem 

may  show  burrow  of  the  stem  borer,  or  possibly  the  plant 

is  cut  at  soil  level  by  cutworms. 

Borer,  cutworms,  etc.  (Text  fig.  5  and  p.  30). 
Tuber  diseases. 

Tuber  rots. 

Field  rots. 

Burrows  with  worms  inside  and  soft  ill-smelling  rot, 
following  foliage  infestation  by  leaf-mining  worm. 
Tuber  moth  (PI.  XI  and  p.  29). 
Brown  dry  rot  running  just  under  surface  of  tuber, 
which  often  has  a  purple  tinge.  This  is  the  late 
blight  rot,  usually  followed  by  various  soft  rot 
organisms;  it  occurs  in  heavy,  cold,  wet  soils 
when  late  blight  has  attacked  the  tops. 

Late  blight  rot  (PI.  VIII  and  p.  28). 
132864°— 19 2 


18  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

Tuber  diseases — Continued. 

Tuber  rots — Continued. 

Storage  rots.  Wet  and  dry  rots  of  various  appearances,  often 
with  inyeeliuni-lined  cavities  and  frequently  rather  soft 
and  watery -brownish.  These  are  the  Fusarium  rots  which 
follow  wilt  infection  of  plants  and  most  commonly  start 
at  the  stem  end.  Slight  wounds  also  furnish  entrance  to 
the  several  wound-invading.  Fusarium-rotting  organisms. 

Fusarium  rots  (p.  28). 
Surface  blemishes. 

Rough,  corky  scab  spots  scattered  over  surface  or  covering  it 
entirely. 

Corky  scab  (Text  figs.  1  and  2  and  p.  26). 
Small  black  raised  masses  resembling  dirt  on  skin ;  not  readily 
washed  off  but  easily  scraped  off  with  the  nail.  Badly  at- 
tacked tubers  roughened  and  cracked  open.  A  trouble 
occurring  simultaneously  with  rosette  and  caused  by  the 
same  organism. 

Black  scurf  or  russet  scab  (PI.  Till  and  p.  24 L 
Numerous  small  galls  and  pimples  on  the  surface  of  the  tuber. 
Within  the  galls  are  minute,  pearly-white,  glistening 
bodies.  If  the  tuber  be  cut  a  row  of  these  glistening 
bodies  may  be  traced  about  one-eighth  inch  under  the  peel. 
Affected  tubers  rot  quickly. 

Nematode  or  gallworm  (PI.  XIV  and  p.  33). 

PARASITIC  DISEASES. 

Fusarium   Wilt    (Fusarium    oxysporum). 

The  wilt  disease  of  Irish  potatoes  caused  by  the  fungus  Fusarium 
o.ryspot  urn  is  prevalent  in  the  principal  sections  of  the  islands  where 
this  crop  is  grown.  Potatoes  from  California,  both  seed  and  table 
stock  often  used  here  for  seed,  are  frequently  found  infected  with 
this  disease.  It  is  the  most  serious  and  persistent  disease  with  which 
the  growers  of  that  State  have  to  contend,  and  it  is  highly  probable 
that  the  disease  was  introduced  into  Hawaii  on  seed  from  California. 

The  disease  is  characterized  by  a  slight  rolling  and  wilting  of  the 
leaves  and  premature  drying  of  the  plants.  The  lower  leaves  wilt, 
droop,  and  die  first,  and  the  color  of  the  foliage  in  the  early  stages 
may  be  lighter  than  usual.  As  a  rule  the  disease  is  not  evident  until 
the  plants  are  about  a  foot  or  more  high.  The  appearance  of  the 
plants  is  the  same  as  that  of  plants  suffering  from  lack  of  water,  and 
this  is  really  the  case,  even  though  there  is  plenty  of  water  in  the 
soiL  Upon  splitting,  the  woody  portion  of  the  underground  stem 
is  seen  to  be  discolored,  that  is,  more  or  less  browned  (PL  I,  fig.  2). 
The  fungus  Fusarium  oxysporum  enters  the  plant  either  from  the 
seed  or  from  the  soil  through  the  smaller  roots  and  works  its  way 
up  through  the  water-carrying  vessels  of  the  roots  and  stem,  and 
often  later  through  the  stolons  into  the  young  developing  tubers 
(PI.  I,  fig.  1).  The  fungus  mechanically  obstructs  the  water  ducts 


POTATO   DISEASES   IN   HAWAII.  19 

or  vascular  system  and  limits  the  amount  of  water  available  to  the 
plant.  Whether  there  are  other  injuries  to  the  plant  through  se- 
cretions of  harmful  products  by  the  fungus  has  not  been  demon- 
strated. 

The  wilt  disease  results  when  seed  carrying  the  disease  is  used  or 
when  healthy  seed  is  planted  upon  soils  already  infected  with  the 
parasite.  With  continuous  cropping  the  disease  becomes  more  ser- 
ious from  year  to  year  until  finally,  unless  seed  selection  and  crop 
rotation  are  practiced,  potatoes  can  no  longer  be  profitably  grown. 
Some  of  the  fields  in  Maui  appear  to  have  reached  this  stage,  partly 
from  soil  exhaustion.  The  plants  die  and  dry  up  shortly  after 
blossoming,  and  the  tubers  remain  small  and  only  partly  developed. 

The  fact  that  Fusarium  wilt  may  be  readily  overlooked,  the  prema- 
ture drying  of  the  plants  being  mistaken  for  the  natural  result  of 
prevailing  weather  conditions,  constitutes  an  insidious  danger,  since 
diseased  plants  yield  a  preponderance  of  small  unmarketable  tubers, 
which,  according  to  the  Hawaiian  method  of  marketing,  are  kept 
for  seed,  the  large  and  medium  sized  tubers  being  sold.  The  result 
of  this  practice  and  of  continuous  cropping  is  that  only  in  an 
occasional  season  can  anything  like  a  satisfactory  crop  be  raised  in 
fields  where  this  disease  is  established. 

Control. — The  control  measures  which  have  been  suggested  for 
this  disease  consist  chiefly  of  seed  selection  and  crop  rotation. 
Neither  of  these  measures  will  entirely  eliminate  the  wilt  fungus, 
nor  is  it  probable  that  a  rotation  of  less  than  three  years  will  ap- 
preciably improve  badly  infected  fields,  but  it  is  expected  that  in 
the  absence  of  susceptible  crops  for  a  three-year  period  the  virulence 
of  the  disease  will  be  reduced  and  one  crop  of  potatoes  can  then  be 
grown  profitably  provided  healthy  seed  be  used. 

Seed  from  disease-free  fields  should  be  obtained  whenever  possible. 
If  imported  seed  is  used,  it  should  be  secured  from  regions  where 
this  disease  is  not  prevalent  and  certified  seed  should  be  insisted 
upon  (see  p.  4). 

Detection  of  wilt  infection  in  seed. — The  following  is  a  useful 
though  not  absolutely  sure  means  of  detecting  the  presence  of  the 
wilt  parasite  in  seed  potatoes.  The  stem  ends  of  a  large  number  of 
the  tubers  to  be  tested  are  cut  across  with  a  knife,  exposing  the  flesh 
within  the  tuber  where  the  woody  fibers  from  the  stolon  (rootlike 
stem  to  which  the  tuber  is  attached  while  growing)  spread  out  to 
form  the  tuber  ring.  If  a  dark-colored  or  brown  ring  appears  or 
brown  fibers  penetrate  the  flesh  at  this  point  (PL  I,  fig.  1),  the  tuber 
should  be  considered  highly  suspicious.  If  many  tubers  show  such 
a  discoloration,  the  whole  lot  should  be  rejected  for  seed  purposes. 
Considering  the  prevalence  of  wilt  infection  in  most  available  seed, 
it  is  advisable  to  cut  off  and  discard  the  stem  ends  of  all  seed  planted. 


20  BULLETIN   45,   HAWAII   EXPERIMENT   STATION. 

This  test  for  wilt  is  not  absolutely  sure,  as  not  all  tubers  carrying 
the  wilt  disease  show  the  discoloration  sufficiently  to  indicate  defi- 
nitely the  presence  of  the  fungus.  Certain  other  diseases  cause  a 
somewhat  similar  discoloration,  and  in  some  cases  tubers  show  a  very 
slight  yellowing  of  the  vascular  ring  which  appears  to  follow  a  long 
period  of  dormancy  and  to  be  unassociated  with  disease.  In  any 
event  these  doubtful  tubers  are  not  desirable  types  to  propagate. 

Late  Blight    (PJiytophtlwra   infestans). 

Xo  disease  of  the  Irish  potato  is  more  destructive  than  the  late 
blight  when  the  conditions  favor  its  development.  Late  blight  is 
caused  by  the  fungus  Phytophthora  infestaiis.  The  name  late  blight 
was  probably  given  to  this  disease  to  indicate  that  it  occurs  most 
commonly  late  in  the  year  and  to  contrast  it  with  the  early  blight 
(Altemaria  solani)  which  is  prevalent  in  the  dry  weather  of  mid- 
summer. It  is  apparent  that  the  common  names  of  these  two  dis- 
eases are  without  significance  in  Hawaii.  The  names  dry  blight  and 
wet  blight  for  early  blight  and  late  blight,  respectively,  are  thought 
to  be  locally  more  appropriate  common  names. 

Wet  blight,  or  late  blight,  is  widely  prevalent  and  destructive  in 
the  Hawaiian  Islands.  It  seems  probable  that  this  disease  and  the 
wilt  disease  {Fusarium  oxysporum)  are  the  most  potent  factors  in 
lessening  yields  and  discouraging  potato  growing  in  these  islands. 
When  the  late  blight  appeared  in  Ireland  in  the  middle  of  the  last 
century  it  so  devastated  the  potato  crops,  upon  which  the  people 
largely  depended  for  food,  that  famine  resulted.  Since  that  time  the 
disease  has  become  prevalent  in  many  of  the  potato  sections  of  the 
world,  and  it  has  been  the  object  of  special  investigation  wherever  it 
occurs.  Various  means  have  been  devised  for  its  control,  but  there 
is  no  ground  for  hope  that  it  can  be  exterminated,  and  potato  growers 
will  probably  always  have  to  contend  with  it. 

The  disease  is  manifested  in  the  leaves  by  dark,  more  or  less 
rounded,  water-soaked  spots  or  areas  (PL  II),  which  may  or  may 
not  increase  rapidly  in  size  and  number,  according  to  weather  con- 
ditions. Upon  the  lower  surface  of  the  leaves  of  affected  plants 
growing  in  damp  situations  a  characteristic  delicate  frostlike  mil- 
dew appears.  This  is  the  parasitic  fungus  bearing  its  microscopic 
spores.  The  latter  are  minute  seedlike  bodies  which,  spattered  about 
by  rain  or  transferred  by  contact  of  the  leaves  with  adjacent  moist 
foliage,  carry  the  disease  from  plant  to  plant.  In  this  way  a  dis- 
eased spot  upon  one  leaf  may  serve  to  infect  a  whole  field  in  an  in- 
credibly short  time.  Similarly  the  spores  falling  and  being  washed 
upon  the  ground  find  their  way  to  the  developing  tubers  which  they 
may  infect,  causing  them  to  rot  in  the  ground  or  subsequently  in 
storage  (PI.  VIII).    With  continued  dry  weather  the  spots  upon  the 


POTATO    DISEASES    IX    HAWAII.  21 

leaves  remain  about  the  same  size  or  spread  slowly,  but  with  heavy 
dews  or  rainy  weather  the  disease  progresses  rapidly.  In  a  few  days 
the  foliage  may  be  all  rotted  away,  the  layman  perhaps  having  over- 
looked the  first  few  spots  and  being  inclined  to  attribute  the  trouble 
to  the  rain  or  the  hot  sun  following  the  rain.  etc.  In  Hawaii,  pro- 
tracted hot  weather  is  sometimes  observed  to  control  the  disease 
completely. 

"Weather  conditions  which  favor  this  blight  are  as  follows: 
High  relative  humidity,  low  soil  temperature,  and  a  high  moisture 
content  of  the  soil.  Since  such  general  conditions  frequently  prevail 
in  the  main  potato  sections  of  Hawaii,  this  disease  may  be  expected 
to  appear  on  almost  every  crop  planted  in  these  localities.  Such 
is  the  rule.  In  some  places  one  crop  a  year  may  suffer  less  than  the 
others.  The  damage  from  this  disease  is  popularly  supposed  to 
be  due  to  the  hot  sun  shining  upon  wet  foliage  following  light 
sprinkling  rains.  There  is  no  question  that  the  major  portion  of 
the  trouble  thus  described  is  the  wet  or  late  blight,  and  not  sunscald 
or  tipburn. 

In  the  islands  the  wet  blight  is  probably  carried  from  crop  to 
crop,  as  these  follow  each  other  almost  continuously  in  the  same  or 
adjacent  fields,  largely  by  volunteer  plants.  Where  any  sort  of 
rotation  is  practiced,  volunteer  potato  plants  are  allowed  to  mature 
and  are  dug.  This  custom  will  have  to  be  discouraged  in  any  at- 
tempt to  control  diseases  and  insect  pests. 

Control. — The  control  measures  worked  out  for  this  disease  con- 
sist entirely  of  methods  of  preventing  infection.  Healthy  and 
vigorous  seed  stock  and  rotation  of  crops  are  important.  Fortu- 
nately, Bordeaux  mixture,  applied  thoroughly  and  in  time  to  the 
foliage,  has  proved  beneficial  in  controlling  this  disease  (Pis.  Ill 
and  IV).  The  insoluble  copper  in  the  mixture  gradually  becomes 
soluble  and  prevents  the  germination  of  the  spores  of  the  fungus 
which  fall  on  the  moist  leaves.  To  be  effective  the  spray  must  cover 
the  entire  surface  of  the  foliage,  and  it  must  be  there  before  the 
spores  find  lodgment.  After  infection  has  taken  place,  the  parasite 
is  out  of  reach  of  the  spray. 

The  practical  arjplication  of  Bordeaux  mixture  to  many  of  the 
larger  fields  is  being  rapidly  worked  out.  In  some  sections  where 
the  blight  is  most  destructive  and  where  the  potato  would  otherwise 
be  at  it>  best,  as  in  the  Glenwood  section  of  Hawaii,  the  frequent 
and  excessive  rains  make  thorough  and  timely  -praying  an  uncer- 
tain and,  for  the  most  part,  an  impossible  matter.  Even  under  these 
conditions,  spraying  may  afford  some  relief  when  the  weather  con- 
ditions are  such  as  to  permit  the  -pray  to  dry.  The  Hamakua 
Hybrid  potato  promises  to  resist  the  blight  under  the  conditions  at 


22  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

Glenwood,  and  there  is  every  indication  that  this  variety  may  solve 
the  blight  problem  for  such  districts. 

The  spraying  with  Bordeaux  mixture  thus  far  carried  on  by  the 
writer  and  his  associates  for  the  prevention  of  wet  blight  or  late 
blight  has  resulted  in  an  increase  in  the  crop  of  from  50  to  200  per 
cent  by  weight.     (See  Pis.  Ill  and  IV.) 

Resistant  varieties. — For  many  years  attention  has  been  directed 
to  the  development  of  varieties  resistant  to  the  late  blight  in  Europe 
and  on  the  mainland  of  the  United  States,  but  thus  far  without 
striking  success.  Varieties  showing  resistance  have  been  types  not 
in  demand  on  the  American  markets. 

A  type  of  potato  originated  by  Miss  Yamata,  of  Honokaa,  Hawaii, 
is  reported  to  be  much  more  successfully  grown  in  that  locality  than 
any  other  sort.  This  type  of  resistant  potato,  of  which  there  are 
two  strains,  has  received  the  name  Hamakua  Hybrid  from  the  dis- 
trict in  which  it  first  came  to  the  attention  of  the  station.  One 
parent  of  both  strains  appears  to  have  been  the  local  variety  known 
as  the  Portuguese  Purple,  originally  brought  to  the  islands  by 
Portuguese  immigrants.  One  form  of  the  Hybrid  is  a  white  and 
purple  mottled  tuber,  of  the  general  shape  of  a  Burbank,  with  rather 
deep  sprouts  and  eyes.  The  other,  possibly  a  cross  between  the 
Portuguese  Purple  and  the  Early  Eose,  has  a  pink  spin  and  purple 
eyes  and  sprouts,  and  is  a  more  desirable  tuber  for  market. 

Practical  results  support  the  view  that  this  Hybrid  has  a  promis- 
ing degree  of  resistance  to  late  blight  in  these  islands,  but  experimental 
evidence  is  somewhat  conflicting.  At  Glenwood  two  experiments  by 
different  workers  have  shown  its  practical  value  in  that  district. 
Four  acres  of  the  variety  grown  in  this  section  yielded  over  400  bags 
of  100  pounds  each.  At  Castner  a  small  patch  of  the  Hybrid  and  the 
Portuguese  Purple  varieties  planted  October  20,  1917,  was  unaffected 
by  late  blight  on  February  1,  1918,  when  other  potatoes  planted  at 
the  same  time  within  a  few  hundred  feet  were  devastated  by  the 
disease.  The  grower  stated  that  Burbank  potatoes  planted  at  the 
same  time  as  the  Hybrid  and  adjoining  them  were  all  dead  in  60 
days.  The  Hybrid,  therefore,  grew  through  a  period  of  more 
than  three  months  of  weather  suitable  for  late  blight.  They  showed 
some  early  blight  spots  but  no  trace  of  the  late  blight  disease. 

In  order  to  test  the  blight  resistance  of  the  Hamakua  Hybrid,  it 
was  also  grown  in  comparison  with  the  Early  Rose  variety  in  a  small 
plat  at  Castner  substation  (PI.  V).  Owing  to  the  poor  soil  there, 
the  yield  of  neither  variety  is  normal,  but  the  resistance  of  the  Hama- 
kua Hybrid  to  late  blight  is  striking.  Both  varieties  grew  well  the 
first  month,  but  at  the  end  of  60  days  the  tops  of  the  Early  Rose  had 
been  destroyed  by  the  late  blight  (PL  V),  the  96  hills  yielding  only 
TJ  pounds  of  tubers  the  size  of  marbles.    The  Hamakua  Hybrid  re- 


POTATO   DISEASES   IN    HAWAII.  23 

mained  almost  without  a  trace  of  blight  spots,  and  at  the  end  of  the 
fourth  month  20  hills  were  dug,  yielding  9-J  pounds  of  fair-sized 
tubers.  The  Hamakua  Hybrid  grew  through  weather  favorable  to 
blight  for  most  of  the  period  of  the  experiment. 

It  appears  from  the  foregoing  that  the  Hamakua  Hybrid  and  one 
of  its  parents,  the  Portuguese  Purple,  possess  a  real  resistance  to 
the  late  blight  under  Hawaiian  conditions.  Should  the  indicated  re- 
sistance to  late  blight  of  the  Hamakua  Hybrid  be  of  more  than  local 
importance,  the  quality  of  the  tubers  can  doubtless  be  improved  by 
selection  in  the  direction  of  market  requirements,  though  there  is  not 
the  discrimination  here  against  colored  and  long  potatoes  that  is 
characteristic  of  the  eastern  markets. 

Early   Blight  or  Dry   Blight  (Alternaria  solani). 

Early  or  dry  blight,  a  fungus  disease  (PL  VI)  attacking  only  the 
foliage  of  the  potato  and  producing  a  characteristic  spotting  and 
death  of  the  leaves,  is  prevalent  in  the  Territory  in  dry,  hot  seasons 
and  occasionally  may  cause  a  loss  of  from  5  to  25  per  cent  of  the  crop 
in  affected  fields.  As  previously  noted,  the  name  "  early  blight "  as  ap- 
plied to  this  disease  on  the  mainland  has  little  significance  in  Hawaii, 
unless  it  be  that  the  disease  attacks  the  plants  earlier  in  their  period  of 
growth  than  is  characteristic  of  the  late  or  wet-weather  blight.  As 
previously  noted,  the  name  dry  blight  or  dry-weather  blight  is  locally 
more  appropriate  for  this  disease. 

The  parasitic  fungus  produces  circular  to  more  or  less  angular, 
dry,  brown  leaf  spots,  often  with  concentric  markings  with  something 
of  the  appearance  of  a  target  (PI.  VI).  The  spots  are  from  one- 
sixteenth  to  one-fourth  inch  in  diameter  and  sometimes  confluent,  and 
frequently  the  dry  tissue  falls  out,  giving  a  shot-  hole  effect  to  the 
leaves.  The  spread  of  the  spot  may  be  limited  in  certain  directions 
by  the  leaf  veins,  in  which  case  it  is  more  or  less  angular  in  shape. 
The  spots  caused  by  the  dry  blight  are  from  the  first  dry  and  brown 
and  the  progress  of  the  disease  is  comparatively  slow.  The  badly 
affected  leaves  dry  and  die  after  a  few  weeks.  The  stems  and  tubers 
are  not  directly  affected,  but  as  a  result  of  defoliation  and  early 
maturity  of  the  plant  the  yield  is  materially  reduced. 

Early  or  dry  blight  appears  to  be  only  occasionally  serious  in  the 
islands.  The  prolonged  drought  during  the  summer  and  fall  of  1917 
in  the  Hamakua  district  of  the  island  of  Hawaii  was  particularly 
favorable  to  its  development.  A  loss  of  25  per  cent  was  observed 
in  some  cases,  most  of  it  due  apparently  to  this  disease,  though  the 
mite  disease,  subsequently  to  be  discussed,  and  bud  and  leaf  infesta- 
tion with  the  tuber  moth  were  undoubtedly  responsible  for  a  portion 
of  the  damage. 


24  BULLETIN  45,    HAWAII   EXPERIMENT   STATION. 

Control. — The  experience  of  investigators  on  the  mainland  has 
shown  that  dry  blight  can  be  controlled  by  timely  and  thorough 
spraying  with  Bordeaux  mixture.  Where  the  conditions  are  fa- 
vorable to  the  development  and  spread  of  this  malady,  the  plants 
should  be  sprayed  as  recommended  for  the  late  or  wet  blight.  As 
a  general  practice,  potato  plants  in  the  islands  should  be  sprayed 
from  three  to  six  times  per  crop  with  Bordeaux  mixture,  fewer- 
sprayings  being  required  in  dry  seasons  than  in  wet  seasons.  In  the 
former  case  the  spray  will  prevent  the  attacks  of  the  early  or  dry 
blight  and  in  the  latter  the  attacks  of  the  late  or  wet  blight. 

Black  Scurf  (Russet  Scab)  axd  Rosette  (Rhizoctonia  solani). 

The  diseases  of  the  potato  commonly  attributed  to  the  parasitic 
fungus  Ehizoctonia  are  quite  diverse  in  their  general  aspects  or 
signs,  but  they  are  conveniently  grouped  together  for  discussion. 
The  Ehizoctonia  fungus  is  a  soil  inhabitant  which  attacks  the  under- 
ground stem  and  roots  of  the  plant  (PL  VII)  with  effects  varying 
according  to  the  portion  injured. 

Black  scurf  is  the  name  commonly  applied  to  the  black  accumu- 
lated mycelial  masses  or  sclerotia  of  the  fungus  which  adhere  closely 
to  the  skin  of  the  tuber  in  such  a  way  as  to  be  readily  mistaken  for 
bits  of  soil.  (See  PI.  VIII.)  They  do  not  wash  off  easily,  although 
they  are  superficial  and  readily  scraped  off  with  the  finger  nail. 
This  stage  of  the  fungus  does  little  damage  other  than  to  the  appear- 
ance of  the  tuber,  but  it  is  hy  this  resisting  stage  of  the  parasite  that 
the  disease  is  largely  spread  to  new  land  and  to  new  localities. 

If  tubers  affected  with  the  black  scurf  are  planted,  the  sprouts 
may  be  attacked  and  girdled  by  the  vegetative  threads  of  the  fungus. 
As  a  result  of  this  action  the  following  types  of  disease  may  occur : 

Weak  plants :  The  fungus  kills  or  injuries  the  sprouts  and  a  poor  stand 
and  weak  plants  result  (PI.  VII,  fig.  1). 

Rosette  disease  and  leaf  roll:  The  leaves  of  the  plant  become  light  green 
and  roll  upward  on  the  midrib  as  an  axis,  becoming  tubular  (PI.  IX).  The 
plants  may  be  more  or  less  dwarfed.  The  few  undersized  tubers  which  form 
are  set  close  to  the  stem,  often  pushing  out  of  the  ground  (PI.  VII,  fig.  2). 

Aerial  tubers :  The  stem  below  ground  is  girdled  more  or  less  completely. 
The  food  elaborated  in  the  leaves  and  designed  for  subterranean  tuber  forma- 
tion is  diverted,  and  tubers  form  above  ground  in  the  leaf  axils  (PI.  X). 

Growth  to  top:  The  stolons  or  stems  of  the  young  tubers  are  attacked  by 
the  fungus  and  the  young  tubers  being  partially  cut  off  from  nourishment  fail 
to  develop  normally.  The  plant  nfay  develop  luxuriantly  and  yield  only  a  few 
small  tubers. 

Disfigured  tubers:  The  tubers  are  frequently  covered  with  the  black  scurf 
or  sclerotial  resting  stage  of  the  fungus.  The  same  stage  of  the  fungus  is  be- 
lieved commonly  to  be  responsible  for  rough  brownish  and  russet  areas  on  the  sur- 
face of  the  tubers.  Throughout  such  areas  the  skin  is  finely  and  irregularly 
cracked.  Large  cracks  in  tubers  are  also  attributed  to  the  action  of  this  organism 
as  well  as  to  uneven  growth,  and  deep  pit-like  holes  often  occur  which  may  he 
caused  by  this  fungus  (PI.  VIII). 


POTATO   DISEASES   IN    HAWAII.  25 

The  name  Rhizoctonia,  as  used  here,  is  a  group  name  for  a  number 
of  closely  related  forms.  This  sort  of  fungus  is  a  soil  inhabitant 
which  is  thought  to  be  favored  by  heavy,  wet,  sour  soils,  but  in  these 
islands  it  appears  to  be  responsible  for  considerable  damage  even 
on  porous,  light,  well-drained  soils. 

Control. — The  measures  found  effective  in  controlling  this  soil 
organism  are  seed  selection  and  seed  disinfection  in  corrosive-subli- 
mate solution  (p.  9),  combined  with  planting  in  uninfected  soils  or 
soils  in  which  the  liability  of  infection  has  been  reduced  by  crop 
rotation.  Root  crops  such  as  turnips  and  beets  are  susceptible  to  the 
parasite  and  should  not  be  used  in  the  rotation.  The  Rhizoctonia 
types  of  injury  are  very  prevalent  in  Hawaiian  fields,  and  attention 
to  the  control  of  these  diseases  will  have  to  be  given  before  any  great 
increase  in  yield  can  be  had  in  some  localities. 

Sclerotium  Wilt  {Sclerotium  rolfsii). 

The  Sclerotium  wilt  disease  is  caused  by  the  fungus  Sclerotium 
rolfsii,  described  by  Rolfs *  as  the  cause  of  a  wilt  of  tomato  in  Florida. 
Besides  the  tomato,  this  fungus  causes  a  similar  disease  of  the  fol- 
lowing plants:  Potato,  peanut,  eggplant,  bean,  cowpea,  summer 
squash,  cabbage,  beet,  and  melon.  It  is  reported  to  be  very  destructive 
to  the  Irish  potato  in  heavy,  wet  soils.  This  fungus  has  been  isolated 
from  diseased  peanuts  growing  on  the  farm  of  the  College  of  Hawaii, 
Oahu,  and  also  from  Hilo,  Hawaii,  but  as  yet  Irish  potatoes  attacked 
by  this  disease  in  the  islands  have  not  come  under  the  writer's  ob- 
servation. Larsen  describes  it  as  occurring  on  potatoes  in  the  vicinity 
of  Honolulu  in  1913.  He  isolated  the  fungus  and  performed  suc- 
cessful inoculations  on  potato  plants.  Since  his  publication  is  not 
generally  available,  and  this  disease  is  likely  to  be  of  occasional  im- 
portance in  some  localities  on  the  islands,  the  following  quotation 
from  his  description  is  given : 

Sometime  last  January  (1913)  our  attention  was  called  to  a  field  of  po- 
tatoes in  the  vicinity  of  Honolulu  which  was  being  entirely  destroyed  as  a 
result  of  some  disease  while  the  plants  were  still  immature.  On  investigation 
it  was  found  that  the  field  in  question,  covering  some  2  acres,  was  affected 
with  a  fungus  malady  known  to  mycologists  as  sclerotial  disease.  The  same 
trouble  was  then  found  in  other  potato  patches  about  Honolulu  and  occasion- 
ally on  other  host  plants  as. well.  In  most  cases  the  trouble  was  fatal  to  the 
affected  plants,  causing  severe  loss  wherever  it  occcurred. 

The  first  indication  of  sclerotial  disease  is  a  slight  drooping  of  some  of  the 
younger  leaves  and  leaf  tips.  On  succeeding  days  the  wilting  becomes  more 
pronounced,  until  in  the  course  of  two  to  four  days  the  entire  shoot  wilts  and 
fails  to  recover.  *  *  *  Sometimes  several  or  all  the  shoots  wilt  simultane- 
ously. At  the  first  indication  of  wilt,  if  one  examines  the  base  of  an  affected 
shoot,  just  below,  and  sometimes  also  a  little  above,  the  surface  of  the  soil,  one 
will  find  that  the  outer  tissue  through  the  cambium  is  decayed.  *  *  *  The 
decayed  area  may  or  may  not  extend  clear  around  the  stem.     Eventually  the 

1  Rolfs,  T.  H.     Tomato  diseases.     Florida  Sta.  Bui.  117  (1913),  p.  40. 


26  BULLETIN   45,    HAWAII   EXPERIMENT  STATION. 

fungus  travels  inward,  past  the  woody  bundles  and  across  the  entire  shoot, 
which  then  falls  over  as  if  broken  at  the  base.  Even  before  this  collapse  takes 
place  the  disease  will  have  penetrated  beyond  the  woody  vascular  tissue  into 
the  soft  and  watery  core,  in  which  it  travels  upward  and  downward  with  great 
rapidity.  The  core  tissue  is  quickly  discolored  and  soon  decays,  leaving  the 
woody  circle  of  vascular  tissue  like  a  hollow  skeleton  of  the  original  shoot. 
During  moist  weather  conditions,  a  white  mycelial  growth  may  be  seen  over 
the  affected  tissue  and  in  the  hollow  stem,  and  white  mycelial  strands  may 
occur  in  the  soil  surrounding  the  plant.  This  mycelium  soon  produces  small 
white  tufts,  which  become  round,  smooth,  and  hard,  and  change  from  white  to 
yellow,  then  finally  to  dark  brown.  Such  hardened  mycelial  masses  are  known 
as  sclerotia.  They  are  the  only  reliable  means  of  identifying  the  disease. 
When  mature  and  dry  they  resemble  mustard  seed  both  in  size  and  color. 
During  dry  weather  these  sclerotia  do  not  develop  under  natural  conditions, 
and  artificial  means  may  be  adopted  to  induce  their  formation  in  order  to 
diagnose  the  disease.  This  can  be  done  by  placing  one  of  the  affected  shoots 
in  a  moist  chamber     *     *     *. 

Larsen  records  alfalfa,  coreopsis,  taro,  and  Irish  potato  as  affected 
with  sclerotium  in  these  islands. 

Control. — Rolfs1  found  the  Sclerotium  disease  of  tomatoes  sus- 
ceptible of  treatment  with  some  soluble  form  of  fungicide  such  as 
an  ammoniacal  solution  of  copper  carbonate  or  eau  celeste.  He 
writes  as  follows  regarding  this  means  of  treatment,  which  is  very 
successful  in  Florida: 

One  of  these  fungicides,  preferably  the  ammoniacal  solution  of  copper  car- 
bonate, should  be  sprayed  on  the  soil  about  the  stem  of  the  plant.  By  spraying 
on  a  half  teacupful  at  this  point  the  plant  is  usually  perfectly  protected  against 
infection.  In  using  this  remedy  it  should  be  remembered  that  where  the 
fungus  has  gained  entrance  into  the  tissues  of  a  plant  before  the  fungicide 
has  been  applied,  the  remedy  will  be  of  no  avail. 

The  formula  and  description  of  preparation  of  the  two  fungicide 
solutions  is  given  on  pages  13  and  11.  As  an  additional  means  of 
controlling  the  ravages  of  the  fungus,  the  soil  about  the  plants  should 
be  loosened  and  exposed  to  the  action  of  the  sun  and  air.  In  dry 
weather  this  serves  to  kill  the  fungus  to  some  extent,  thus  preventing 
infection. 

Common  or  Cokky  Scab    {Actinomyces  chromogenus). 

The  disease  known  as  common  or  corky  scab  attacks  the  growing 
tubers  at  any  stage  of  development  (figs.  1  A  and  2  B).  The  centers 
of  infection  are  first  evident  as  small  brownish  spots  on  the  sur- 
face of  the  tuber.  The  spots  increase  rapidly  in  size  and  depth 
until  the  whole  surface  may  become  covered  with  the  unsightly, 
rough,  scabby  areas.  In  some  cases  the  potatoes  crack  open  or  the 
spots  are  enlarged  and  deepened  by  insects.  The  scabs  consist  of 
accumulated  corky  tissue  formed  by  the  tuber  in  an  attempt  to  pro- 
tect the  underlying  tender  tissues  from  the  irritating  action  of  the 

i  Rolfs,  P.  H.    Tomato  diseases.     Florida  Sta.  Bui.  117  (1913),  p.  41. 


POTATO   DISEASES   IX    HAWAII.  27 

encroaching  fungus.  This  disease  is  caused  by  a  bacterial-like  or- 
ganism until  recently  known  under  the  name  Oospora  scabies,  but 
its  relationship  to  another  group  of  fungi  having  been  shown,  it  is 
now  called  Actinomyces  ehromogenus. 

The  corky  scab  disease  not  only  reduces  the  value  of  the  crop  but 
also  appreciably  lessens  the  yield.  The  losses  through  decreased 
yield  have  been  estimated  variously.  For  example,  Goff1  planted 
equal  amounts  of  very  scabby  seed  and  seed  free  from  scab  on  a  soil 
which  had  been  in  clover  sod.  The  scabby  seed  yielded  199f  pounds 
and  the  scab-free  seed  477§  pounds.  The  decreased  yield  is  brought 
about  through  the  failure  of  the  seed  to  germinate  and  the  lowered 
vitality  of  the  plants.  The  value  of  the  crop  is  reduced  in  several 
ways.  The  unsightliness  of  scabby  potatoes  and  their  changed  taste 
and  odor  lower  their  market  value.  The  increased  liability  to  tuber 
decay  and  the  the  extra  thick  paring  necessary  in  preparing  the 
potato  for  the  table  result  in  considerable  loss  to  the  consumer. 

This  disease  is  not  very  prevalent  in  the  islands,  but  at  times  on 
poorly  drained,  hard-packed  soils  or  soils  which  have  been  limed 
it  is  more  or  less  troublesome  and  probably  the  source  of  considerable 
loss. 

Control. — Seed  selection  and  disinfection,  certain  types  of  ferti- 
lization and  soil  treatment,  and  crop  rotation  have  been  found  more 
or  less  successful  in  preventing  the  common  or  corky  scab.  The 
organism  causing  the  disease  is  distributed  for  the  most  part  by  the 
use  of  scabby  seed  potatoes,  and  only  scab-free  tubers  should  be  used 
for  seed.  To  reduce  the  probability  that  scab  organisms  are  carried 
to  the  soil  by  the  seed,  the  latter  should  always  be  dipped  in  one  of 
the  potato-seed  disinfecting  solutions  (p.  9). 

Scabby  and  refuse  potatoes  should  not  be  fed  to  farm  animals  if 
the  manure  from  these  animals  is  to  be  scattered  on  the  potato  land, 
as  it  has  been  demonstrated  that  the  organism  can  survive  passage 
through  the  alimentary  tract  of  animals. 

The  development  of  the  scab  is  favored  by  a  high  humus  content  of 
the  soil  and  also  by  an  alkaline  reaction.  To  the  latter  fact  may  be 
attributed  the  unfavorable  results  of  adding  lime  or  wood  ashes  to 
potato  soils.  Halsted  2  obtained  favorable  results  by  fertilizing  with 
kainit  (Stassfurt  salt  containing  20  per  cent  potash).  According  to 
Beckwith 3  and  Taft 4  muriate  and  sulphate  of  potash  also  reduce  scab 
injury.  The  plowing  in  of  green  manure  and,  in  fact,  any  cultural 
practice  which  tends  to  make  the  soil  acid  will  reduce  the  common 
or  corky  scab  on  badly  infected  fields.      Crop  rotation  has  been  sug- 

1  Goff,  E.  S.     Experiments  in  potato  culture.     Wisconsin  Sta.  Rpt.  1892.  pp.  278-280. 
-  Halstead.  B.  D.     Field  experiments  with  potatoes,  I.     New  Jersey  Sta.  Bui.  112  (1S95), 
pp.  1-12;  Rpt.   1S96.  pp.   309-315. 

3  Beckwith,  M.  H.     Potato  scab.     New  York  State  Sta.  Rpt.  1SS7,  pp.  307-315. 

4  Taft,  L.  R.     [Potato  scab.]     Michigan  Sta.  Bui.  57  (1890),  pp.  23-25. 


28  BULLETIN  45,   HAWAII   EXPERIMENT   STATION. 

gested  with  the  assumption  that  in  the  absence  of  the  potato  for  a 
number  of  years  the  organism  would  die  out.  Inasmuch  as  turnips, 
beets,  and  other  root  crops  are  susceptible  to  attack,  they  should  not 
be  included  in  the  rotation,  as  they  would  serve  to  carry  over  the 
disease. 

Potato  Tuber  Rots   (Phytophthora  infestans,  Fasarium  oxpsporum, 
F.  radidcolcL,  and  F.  coeruleum). 

In  spite  of  the  fact  that  the  major  part  of  the  potato  crop  of  the 
islands  is  consumed  soon  after  the  tubers  are  dug,  the  total  annual 
loss  by  rotting  is  an  item  of  considerable  importance.  It  is  a  serious 
source  of  trouble  in  keeping  seed  stock  and  often  results  in  the  use 
of  inferior  tubers  for  this  purpose. 

The  type  of  rot  most  commonly  observed  is  often  called  dry  rot, 
though  this  name  is  frequently  misleading,  as  the  nature  of  the  rot 
depends  upon  the  amount  of  moisture  and  upon  the  temperature. 
With  plenty  of  moisture  and  a  moderately  warm  temperature  the 
rot  develops  rapidly  and  is  of  the  soft  type.  If  the  rot  develops 
more  slowly,  or  if  wet  rotting  tubers  dry  out,  the  typical  dry-rot 
phase  is  produced.  These  forms  of  rot  are  caused  for  the  most 
part  by  three  species  of  Fusarium  known  as  F.  oxysporum,  F.  radi- 
cicola,  and  F .  coeruleum.  Hotting  of  the  tubers  is  very  prevalent  in 
the  crop  from  wilt-infected  vines. 

Another  tuber  rot  is  caused  by  the  late  blight  fungus.  When  the 
tops  are  destroyed  by  this  blight,  the  tubers  may  rot  badly  in  the 
ground.  The  spores  of  the  fungus  are  washed  from  the  leaves  to 
the  ground  and  thence  to  the  tubers,  which  are  infected.  This  rot 
is  a  firm  brown  discoloration  extending  through  the  surface  layers  of 
the  tuber  a  short  distance  beneath  the  skin.  In  the  earliest  stages 
it  shows  through  the  skin  as  a  slight  brownish  or  purplish  area,  which 
later  becomes  more  or  less  shrunken.  In  heavy,  cold,  wet  soils  this 
rot  spreads  rapidly  through  the  tuber,  and  a  soft,  ill-smelling,  second- 
ary rot  caused  by  bacteria  hastens  the  destruction.  In  drier  soils 
little  evidence  of  rot  may  be  present,  and  yet  the  tubers,  if  infected, 
will  rot  badly  in  storage. 

Control. — The  rotting  of  potatoes  can  be  largely  controlled  by 
storage  at  a  temperature  of  about  35°  F.,  and  where  low  temperature 
facilities  are  at  hand  such  storage  offers  a  practical  means  of  con- 
serving the  potato  crop.  However,  even  at  this  temperature  the  rots 
progress  slowly,  and  for  best  results  the  tubers  should  be  carefully 
sorted  before  being  put  into  storage.  Storage  facilities  at  such  a 
temperature  are  rarely  available  to  Hawaiian  growers,  and  at  pres- 
ent little  advantage  can  be  taken  of  this  method  of  holding  the  crop 
or  seed  tubers.  To  prevent  loss  through  rotting  under  conditions  in 
the  islands,  attention  should  be  directed  toward  the  prevention  of  in- 


POTATO   DISEASES   IN    HAWAII. 


29 


fection  through  the  use  of  good  seed,  crop  rotation,  control  of  dis- 
eases and  insect  pests  as  far  as  possible,  careful  handling  of  the 
crop,  and  storage  at  the  lowest  available  temperature. 

Tuber  Moth  (Phthorimoea  operculella) . 

The  tuber  moth  is  the  most  serious  insect  pest  of  the  Irish  potato 
in  Hawaii  because  of  the  damage  it  does  and  the  difficulty  with  which 
it  can  be  controlled.  It  ranks  second  only  to  late  blight  as  a  poten- 
tial pest  of  this  crop,  though  the  conditions  favorable  to  the  one  are 
unfavorable  to  the  other.  This  insect  also  attacks  tomato,  eggplant, 
and  tobacco  (as  splitworm),  though  less  actively  than  it  does  the 
potato. 

The  moth,  which  is  small  and  grayish  in  color  (fig.  4),  lays  its 
eggs  upon  the  leaves  or  other  parts  of  the  plant.     The  larva?  or 

worms  quickly  enter  the 
leaves,  mining  between 
the  surfaces,  or  bore 
into  the  stem  or  tubers 
(PI.  XI).  Affected 
tubers  soon  rot,  due  to 
invasion  by  one  or  more 
of  the  numerous  wound 
bacteria  or  fungi.  In 
Hawaii  there  are  prob- 
ably several  generations  of  the  pest  during  the  year.  The  moth  is 
most  prevalent  during  the  dry  season,  that  is,  from  May  to  October. 
The  damage  it  does  in  drought  years  is  almost  unbelievable. 

Control. — The  tuber  moth  is  very  difficult  to  control,  even  under 
the  best  systems  of  potato  culture,  and  the  regrettable  lack  of  system 
in  Hawaii  greatly  increases  the  difficulties.  With  potatoes  growing 
in  every  month  of  the  year,  with  practically  no  rotation  in  general, 
and  with  volunteer  potatoes  allowed  to  mature  with  corn  or  beans, 
etc.,  where  an  indifferent  rotation  obtains,  the  outlook  is  not  encour- 
aging. The  procedure  outlined  on  the  mainland  for  the  control  of 
this  pest  is  cited  below,  and  as  far  as  practicable  these  methods  should 
be  followed. 

Clean  cultivation :  All  volunteer  potato  plants  as  well  as  all  solanaceous  weeds 
should  be  pulled  out  and  destroyed.  When  a  potato  crop  is  harvested  the  vines 
and  all  small  tubers  should  be  gathered  up  and  burned. 

Crop  rotation :  Crop  rotation  is  essential,  and  the  cooperation  of  all  growers 
of  a  section  is  necessary.  Crops  which  can  be  used  in  rotation  with  potatoes 
are  beans,  peas,  corn,  cowpeas,  alfalfa,  and  clover. 

Hilling :  Hilling  up  the  potato  plants  much  more  than  is  generally  practiced 
here,  thus  increasing  the  depth  of  the  tubers,  will  probably  help  to  prevent  tuber 
infestation  during  the  maturing  of  the  crop. 

Digging  and  sorting:  The  crop  should  be  dug  as  soon  as  mature  and  brought 
in  from  the  field  before  late  afternoon.     If  the  tubers  are  infested,  as  is  indi- 


Fig.  4. — The  potato  tuber  moth  :  Natural  position  at  rest. 
Much  enlarged.      (From  U.  S.  Dept.  Agr.  Bui.  427.) 


30 


BULLETIN   45,   HAWAII   EXPERIMENT   STATION. 


cated  by  mining  often  visible  just  under  the  skin  and  by  slight  webbing  near 
the  eyes,  they  should  be  carefully  sorted  at  once.  Those  infested  should  be 
either  destroyed  immediately  by  burning  or  fumigated  if  they  are  held  for  use 
as  stock  feed.  Those  still  unimpaired  should  be  fumigated  as  described  below 
and  placed  out  of  reach  of  the  moths  and  of  further  infestation. 

Fumigation :  Place  the  tubers  to  be  fumigated  in  a  room  that  can  be  sealed. 
For  every  1,000  cubic  feet  of  space  to  be  fumigated  place  from  1  to  2  pounds 
of  carbon  bisulphid  in  a  shallow  dish  or  pan  and  set  it  in  the  top  of  the  room. 
The  amount  of  the  chemical  varies  with  the  tightness  of  the  room.  For  small 
containers  the  following  amounts  are  recommended :  Ten-gallon  "  sake  "  barrel, 
one-tenth  to  one-fifth  ounce ;  50-gallon  wine  barrel,  one-half  to  1  ounce.  After  the 
chemical  is  placed  in  the  top  of  the  room  or  container,  seal  up  immediately  and 
leave  undisturbed  for  24  hours.  Remove  and  ventilate  the  material  for  a  short 
time  and  place  in  moth-proof  storage,  if  possible.  If  the  potatoes  are  held  in 
storage  for  any  length  of  time  they  should  be  examined  at  frequent  intervals  for 
further  infestation  and  treated  again  if  necessary.  Seed  potatoes  may  be  fumi- 
gated if  the  lot  is  suspected  of  containing  worm-infested  tubers.  They  must  be 
fumigated  before  the  sprouts  are  prominent,  as  otherwise  the  tubers  will  be 
killed.  Carbon  bisulphid  is  volatile  and  highly  inflammable.  Keep  away  from 
fire.     The  fumes  are  poisonous.    Do  not  inhale. 

Spraying:  The  use  of  Paris  green  or  other  arsenicals  in  Bordeaux  mixture 
(p.  10)  when  the  latter  is  being  regularly  applied  "will  probably  be  of  some 
benefit  in  combating  the  tuber  moth. 

Cutworms,  Army  Worms  or  "  Pokos,"  Snails,  Etc. 

Of  insects  injurious  to  potatoes,  the  cutworms  (fig.  5),  especially 
the  leaf -eating  form  locally  known  as  "  poko,"  rank  next  to  the  tuber 


Fig.  5. — Variegated  cutworm  (Peridroma  margaritosa)  :  a,  Moth ;  h,  normal  form  of 
caterpillar,  side  view ;  c,  same  in  curved  position ;  d,  dark  form,  view  of  back ;  e, 
greatly  enlarged  egg,  seen  from  side;  f,  egg  mass  on  twig.  (From  U.  S.  Dept.  Agr., 
Farmers'  Bui.  739.) 


POTATO   DISEASES   IN    HAWAII.  31 

moth.  The  "  poko "  in  the  early  summer  months  is  a  voracious 
feeder,  and  unless  it  is  checked  by  artificial  means  it  may  entirely 
destroy  the  foliage  of  the  potato  vines.  The  "  pokos  "  as  well  as 
the  true  cutworms  are  night  feeders,  which  during  the  day  may 
usually  be  found  curled  up  in  the  soil  at  the  base  of  the  plant  upon 
which  the}'  are  feeding. 

Control. — The  growers  of  potatoes  in  the  islands  have  been  quite 
successful  in  controlling  these  worms  with  a  poison  bait.  Before 
the  war  it  was  customary  to  use  a  bait  made  of  the  highest  grade 
of  wheat  flour  and  Paris  green,  a  high  grade  of  flour  having  been 
found  economical  by  the  farmers  because  it  is  finer  and  can  be  spread 
more  thoroughly  over  the  vines  and  over  more  plants  than  the 
coarser  grades.  The  poisoned  flour  is  dusted  in  the  late  afternoon 
over  the  plants  and  the  soil  at  the  base  of  the  same.  Some  burn- 
ing of  the  foliage,  or  arsenical  poisoning,  results,  but  this  is  usually 
not  serious  (p.  35).  This  treatment  is  generally  used  and  is  con- 
sidered effective  by  the  growers.  The  cost  of  the  flour  used  is  a 
not  inconsiderable  factor  in  the  expense  of  growing  the  crop.  As 
a  substitute  method  for  the  control  of  these  pests,  the  poison  baits 
of  which  the  formulas  are  given  on  page  1-t  are  now  being  success- 
fully used.  It  seems  probable  that  the  leaf-eating  worms  could  be 
controlled  by  the  use  of  arsenicals  with  Bordeaux  mixture  (p.  13). 
Paris  green  is  used  at  the  rate  of  1  pound  to  50  gallons  of  Bordeaux. 
Where  Paris  green  is  used  the  burning  of  the  foliage  is  prevented 
by  the  life  in  the  Bordeaux,  or,  if  necessary,  an  additional  quantity 
of  lime,  equivalent  to  the  amount  of  Paris  green  used,  may  be  added 
for  the  purpose  of  neutralizing  the  free  arsenious  acid. 

Mite  Disease. 

Irish  potatoes  growing  in  Hawaii  in  dry,  hot  situations,  whether 
irrigated  or  not.  are  frequently  seen  to  dry  up  and  die  from  the 
growing  tip  downward.  The  small  young  leaves  turn  brown  or 
become  bronzed  on  the  under  surface,  get  abnormally  "  fuzzy,'"  and 
twist  or  curl  up,  and  soon  the  shoots  and  leaves  dry  up  and  die 
(PI.  XII).  Often  the  plants  grow  well  until  about  the  time  of 
flowering,  when  they  gradually  dry  up  and  die  prematurely.  The 
yield  of  such  plants  is  negligible.  The  young  growth  is  found  upon 
examination  to  be  attacked  by  a  multitude  of  minute  mites,  which  are 
scarcely  to  be  seen  with  a  hand  lens  having  a  magnification  of  less 
than  20.  These  pests  suck  the  juice  from  the  tender  foliage  and 
ultimately  kill  it. 

This  disease  of  the  Irish  potato  seems  to  be  a  new  one ;  at  least  the 
writer  has  been  unable  to  find  such  a  malady  mentioned  in  the  litera- 
ture.    A   somewhat  related   disease   is  described   under  the   name 


32  BULLETIN   45,    HAWAII   EXPERIMENT  STATION. 

"  phytoptosis  "  on  the  tomato  in  Florida  by  Rolfs.1  The  mite  attack- 
ing the  potato  is  quite  different  from  the  one  found  by  Rolfs,  how- 
ever, this  being  Eriophyes  or  Phytoptus,  while  that  infesting  potatoes 
probably  belongs  to  the  same  group  as  the  so-called  red  spider 
(Tetranychida?). 

The  mite  disease  was  first  noted  by  the  writer  early  in  May,  1917, 
in  the  vicinity  of  Honolulu  and  Castner,  Oahu.  It  was  soon  found 
to  be  very  prevalent  and  destructive  after  the  weather  became  too 
dry  and  warm  for  the  late  blight  disease.  It  is  now  recognized  as  a 
disease  of  considerable  importance  during  dry  seasons  in  the  main 
potato  sections,  and  has  probably  been  destructive  to  the  potato  crops 
for  many  years.  During  the  drought  of  the  summer  and  fall  of  1917, 
this  disease  caused  losses  estimated  at  as  high  as  25  per  cent  in  some 
fields  in  the  Hamakua  district  of  Hawaii  and  the  Makawao  and  Kula 
districts  of  Maui.  Practically  total  losses  were  observed  in  gardens 
in  the  vicinity  of  Honolulu,  where  the  conditions  were  not  favorable 
to  the  crop,  the  weather  being  too  dry  and  warm. 

That  this  disease  is  caused  by  mites  is  established  by  the  following 
facts:  These  organisms  are  always  present  in  sufficient  numbers  on 
plants  with  the  recognized  symptoms  to  be  considered  responsible 
for  the  trouble;  the  reaction  of  the  plant  is  such  as  has  come  to  be 
associated  with  mite  injury;  if  the  mites  are  kept  off  potato  plants 
by  spraying  or  dusting  with  sulphur  the  plants  grow  normally,  while 
adjacent  unprotected  rows  are  devastated;  increases  in  yield  of  tubers 
of  100  per  cent  by  weight  have  been  obtained  by  spraying  with  lime- 
sulphur.     (See  PL  XIII.) 

The  mites  are  oval  shaped  and  almost  colorless  when  young,  be- 
coming slightly  brownish  with  maturity.  When  young  they  have 
three  pairs  of  legs,  later  four  pairs  (PL  XII).  The  eggs,  which  are 
numerous  on  the  affected  leaves,  are  sculptured  or  papillate. 

Control. — Experiments  (PL  XIII)  have  shown  that  the  mite  dis- 
ease can  be  almost  if  not  entirely  prevented  by  dusting  the  plants 
with  sulphur  or  spraying  them  with  a  lime-sulphur  spray.  In  dry, 
warm  weather  watch  the  potato  plants  carefully,  and  as  soon  as  any 
of  the  small  young  leaves  at  the  tip  show  signs  of  turning  brown  or 
becoming  bronzed  on  the  under  surface,  spray  the  entire  surface  of 
the  plants  with  the  lime-sulphur  spray  (p.  13)  or  dust  the  plants 
thoroughly  with  dry  sulphur.    Repeat  as  often  as  necessary. 

The  mite  disease  is  entirely  different  in  its  symptoms  from  the 
late  blight  or  wet  blight  disease  and  likewise  from  the  early  or  dry 
blight,  and  there  should  be  no  confusion  of  these  types.  The  mite 
disease  can  be  confused  with  Fusarium  wilt  or  other  wilt  disease 
unless  careful  examination  is  made.     It  seems  that  there  has  been 

1  Rolfs,  P.  H.  [Tomato  diseases.]  Florida  Sta.  Buls.  21  (1893),  pp.  23,  24  ;  47  (1898), 
pp.  143,  144. 


POTATO   DISEASES   IN    HAWAII.  33 

confusion  of  these  forms,  and  it  is  hoped  that  the  descriptions  of  dis- 
eases herein  will  help  the  grower  to  distinguish  them  in  order  that 
the  proper  treatment  may  be  applied.  Bordeaux  mixture,  which  is  so 
serviceable  as  a  preventive  spray  for  the  two  blights  above  men- 
tioned, will  keep  the  mites  off  the  plants  for  from  one  to  two  weeks, 
but  eventually  sprayed  plants  are  as  badly  affected  as  if  they  had 
received  no  treatment.  Sulphur,  on  the  other  hand,  is  effective 
against  mites,  but  it  is  not  to  be  recommended  for  the  late  blight  if 
it  is  possible  to  use  Bordeaux  mixture. 

Nematodes   (Gall worms  or  Eelworms). 

The  potato  gallworm  or  eelworm,  one  of  a  large  number  of  species 
of  nematodes,  is  a  factor  of  considerable  importance  in  potato  pro- 
duction in  Hawaii.  These  minute  pests  are  not  insects  but  micro- 
scopic round  worms.  The  same  sort  of  parasite  is  responsible  for  the 
rootknot  disease  of  various  crops.  The  gallworm  attacks  the  tubers 
and  causes  the  skin  to  become  roughened  or  cracked  and  covered  with 
irregular  galls  or  pimples  (PI.  XIV,  fig.  1).  Badly  infested  tubers 
shrivel  up,  remain  partly  developed,  and  become  soft  and  otherwise 
unfit  for  table  use.  When  broken  across,  such  tubers  show  a  line  of 
glistening  specks  just  beneath  the  skin  (PI.  XIV,  fig.  2).  These  are 
the  encysted  nematodes. 

There  are  few  cultivated  crops  not  subject  to  attack  by  species  of 
gallworms,  and  when  once  established  in  a  field  their  elimination  is 
practically  impossible.  In  greenhouses  they  can  be  killed  by  steam 
sterilization,  but  in  the  open  field  this  is  seldom  practicable.  They 
are  carried  into  new  soils  by  infested  seed  tubers,  nursery  stock,  etc. 
Infested  seed  potatoes,  which  are  of  course  unfit  for  planting,  con- 
stitute a  serious  menace  to  the  potato  industry. 

Gallworm  injury  to  garden  crops  has  been  observed  on  Hawaii, 
Maui,  and  Oahu.  A  general  infestation  of  fields  with  these  pests 
would  be  disastrous,  and  it  is  essential  that  their  spread  be  limited 
so  far  as  possible.  The  cultivated  fields  of  the  Territory  which  are 
infested  will  be  determined  as  opportunity  permits  and  crops  sug- 
gested for  rotation  where  possible. 

Control. — Seed  potatoes  should  be  carefully  examined  for  the  pres- 
ence of  nematode  galls  before  planting.  As  far  as  possible,  seed  stock 
should  be  secured  from  fields  known  to  be  free  from  this  pest.  Break 
open  any  suspicous  looking  tubers  and  examine  the  outer  third  of  tjie 
flesh  for  minute  brownish  spots  with  pearly  white  centers.  In  case  of 
doubt,  such  tubers  should  not  be  planted,  and  specimens  should  be 
submitted  to  the  station  for  examination. 

There  are  over  500  plants  susceptible  to  the  attacks  of  gallworms, 
including  many  garden  crops.  It  profits  little  to  plant  potatoes  upon 
132864°— 19 3 


34 


BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 


infested  soil,  this  serving  only  to  increase  the  number  of  the  para- 
sites. If  possible,  a  rotation  should  be  practiced  using  nonsusceptible 
crops  where  such  are  suitable.  The  following  susceptible  crops  are 
to  be  avoided: 


Alfalfa. 

Cowpea. 

Okra. 

Soy  bean. 

Asparagus. 

Cucumber. 

Onion. 

Strawberry. 

Bean. 

Eggplant. 

Peach. 

Tobacco. 

Beet. 

Field  pea. 

Pepper. 

Tomato. 

Cantaloup. 

Fig. 

Pumpkin 

Watermelon 

Carrot. 

Grape. 

Rape. 

( telery. 

Kale. 

Salsify. 

Clover. 

Lettuce. 

Spinach. 

There  are  a  few  crops  which  can  be  safely  planted  upon  infested 
soils.  A  three-year  rotation  would  probably  be  the  shortest  that 
would  be  at  all  effective  in  eliminating  the  pests.  At  the  end  of  that 
time  one  planting  of  potatoes  or  other  susceptible  crop  could  prob- 
ably be  made.  Among  the  crops  which  are  not  seriously  affected 
are  the  following: 

Barley.  Peanut.  Rye.  Velvet  bean. 

Corn.  Pearl  millet.  Sorghum.  Wheat. 

Iron  cowpea.  Redtop.  Timothy. 

Corn  and  sorghum  are  particularly  desirable,  as  they  permit  clean 
cultivation  and  the  removal  of  weeds  which  might  harbor  the  pest. 

NONPARASITIC  DISEASES. 

Sux scald,  Sunburn  of  Tubers. 

Under  certain  conditions  of  the  weather  the  leaves  of  the  potato 
plant  may  become  sunburned.  This  condition  may  result  when  the 
sun  comes  out  brightly  after  a  period  of  cloudy  or  misty  weather. 
The  tender  leaves  wilt  and  later  become  dry  and  brown.  This  dis- 
ease has  not  been  found  of  much  consequence  in  Hawaiian  fields. 
Young  plants  frequently  appear  drooping  in  the  middle  of  the  day 
if  the  sun  is  hot  following  showers,  but  little  burning  or  permanent 
injury  to  the  plants  has  been  observed. 

When  growing  tubers  become  exposed  to  the  action  of  the  sun- 
light, they  develop  cholorophyll  (leaf  green)  and  are  said  to  be  sun- 
burned (locally  called  "moonstruck").  As  a  result  of  diseases  like 
Rhizoctonia  root  disease  (p.  24),  which  cause  the  tubers  to  develop  on 
short  stolons  and  in  a  bunch  next  to  the  stem,  the  tubers  are  fre- 
quently forced  out  of  the  soil. 

Control. — Hilling  up  the  plants  and  attention  to  the  control  of 
such  diseases  will  remedy  the  condition. 

Leaf  Tipburn. 

The  disease  characterized  by  the  drying  up,  dying,  and  rolling  up- 
ward of  the  tips  and  the  margins  of  the  leaves  during  a  period  of 


POTATO    DISEASES   IX    HAWAII.  35 

protracted  dry  weather  is  called  tipburn  (PL  XV).  The  older,  or 
lower,  leaves  are  most  affected.  This  trouble  may  possibly  be  mis- 
taken for  early  or  late  blight.  The  absence  of  definite  spots  with  con- 
centric markings  distinguishes  it  from  the  former,  while  the  dry  and 
upward-curling  leaf  margins  of  tipburn  should  serve  to  differentiate 
it  from  the  late  blight.  Tipburn  is  most  common  on  light  soils 
which  lose  moisture  rapidly.  The  disease  results  from  the  fact  that 
under  certain  conditions  the  plant  loses  water  faster  by  transpiration 
(evaporation  from  the  leaves)  than  its  roots  can  absorb  water. 

Control. — Good  surface  cultivation  to  conserve  the  soil  moisture 
and  spraying  with  Bordeaux  mixture  have  been  found  valuable  means 
of  preventing  tipburn. 

Hollow  Potato,  Pkoxged  Potatoes. 

The  peculiarity  known  as  hollow  potato  or  black  heart  occasionally 
occurs  in  Hawaii.  As  the  name  indicates,  the  centers  of  the  tubers 
are  hollow  or  blackened.  This  trouble  is  considered  to  be  due  to 
peculiar  conditions  of  growth.  Uneven  development,  such  as  rapid 
growth  following  a  period  of  cessation  of  growth  or  slow  growth  dur- 
ing drought,  is  thought  to  be  a  factor  in  the  production  of  hollow 
potato  and  is  also  thought  to  encourage  the  formation  of  prongy 
potatoes.  Large,  overgrown  tubers  are  more  frequently  hollow  than 
average  sized  tubers,  and  in  mainland  States  certain  varieties  seem 
to  be  more  commonly  affected  than  others. 

Aksexical  Ixjuby. 

The  application  of  Paris  green  to  the  leaves  of  the  potato  without 
mixing  with  it  any  neutralizing  agent  such  as  lime  often  causes  the 
death  of  a  portion  of  the  leaf  or  of  the  entire  leaf.  Arsenate  of  lead 
is  much  less  active  in  this  way  and  is  to  be  preferred  for  use  on  the 
foliage  of  plants.  In  a  moist  climate,  basic  arsenate  of  lead  is  prob- 
ably the  best  form  to  be  used.  Powdered  arsenate  of  lead  might 
well  be  substituted  for  Paris  green  in  the  flour  poison  bait  used  for 
the  cutworms  and  army  worms  ("poko")  where  the  bait  is  dusted 
over  the  foliage.  If  Paris  green  is  used,  an  equal  amount  of  slaked 
lime  should  be  added  to  prevent  burning. 

The  application  of  arsenicals  in  spray  form  for  pest  control  has 
been  until  recently  but  little  practiced  in  the  islands.  Either  Paris 
green  or  arsenate  of  lead  can  be  used  with  Bordeaux  mixture  with 
little  possibility  of  injuring  the  foliage.  If  Paris  green  rather  than 
arsenate  of  lead  is  used,  a  small  additional  quantity  of  lime  may  be 
added  to  the  Bordeaux  to  neutralize  the  Paris  green.  When  a  water 
suspension  of  Paris  green  is  used  for  spraying,  an  equal  amount  of 
quicklime  should  be  slaked  and  added  to  the  water. 


36  BULLETIN   45,    HAWAII   EXPERIMENT   STATION. 

POTATO  DISEASES  NOT  KNOWN  TO  OCCUR  IN  HAWAII. 

There  are  a  few  very  serious  diseases  and  insect  pests  of  the  Irish 
potato  and  a  number  of  rather  minor  importance  which  apparently 
have  not  yet  become  established  in  Hawaii.  Among  those  which 
have  not  yet  been  observed  are  the  following  parasitic  diseases: 
Black  wart  (which  is  one  of  the  most  destructive  diseases  of  potatoes 
known),  powdery  scab,  blackleg,  southern  bacterial  wilt,  silver  scurf, 
Verticillium  wilt,  and  leak;  and  the  following  supposedly  nonpara- 
sitic diseases  of  unknown  causation :  Internal  brown  spot,  net  necro- 
sis, curly  dwarf,  leaf  roll,  and  mosaic.  Among  the  insects  injurious 
to  the  potato  not  yet  detected  here  are  the  Colorado  potato  beetle 


Fig.  6. — A  potato  affected  with  potato  wart.     (From  U.  S.  Dept.  Agr.,  Farmers'  Bui.  544.) 

and  species  of  flea  beetles.  The  diseases  above  enumerated  are  de- 
scribed in  order  that  they  may  be  recognized  promptly  and  be  the 
better  guarded  against.  It  is  quite  possible  that  some  of  these  dis- 
eases are  already  present  in  the  islands  but  have  not  yet  come  to 
notice.  In  any  cases  where  diseases  are  thought  to  be  like  these 
hereinafter  described,  specimens  should  be  forwarded  without  delay 
to  this  station  for  determination. 

PARASITIC  DISEASES. 

Black  Wart    (Chrysophh/ctis  endobiotica) . 
The  black  wart  disease  was  discovered  upon  potatoes  in  Germany 
less  than  25  years  ago.     Since  its  discovery  it  has  spread  with  great 
rapidity  in  Europe  and  has  recently  appeared  in  Newfoundland.     It 


Bui.  45,  Hawaii  Agr.  Expt.  Station. 


Plate  XV. 


POTATO   DISEASES   IN   HAWAII.  37 

has  very  recently  been  found  in  gardens  in  certain  mining  towns  of 
Pennsylvania,  and  a  survey  of  the  infected  region  is  now  in  progress. 
Black  wart  is  probably  brought  into  new  localities  on  apparently 
healthy  or  only  slightly  diseased  potatoes  coming  from  infected 
fields,  and  it  is  thought  to  have  been  introduced  into  the  United 
States  on  low-grade  European  table  potatoes  before  the  quarantine 
was  established. 

Black  wart  attacks  the  tubers  chiefly  and  causes  malformation 
and  warty  excresences,  converting  the  tuber  into  a  worthless  mass 
(fig  6).  The  warts  have  given  rise  to  such  names  as  warty  disease, 
black  scab,  canker,  and  cauliflower  disease.  Once  infected,  a  soil 
can  not  be  used  for  this  crop  again  for  many  years,  unless  resistant 
varieties  are  found,  as  now  seems  possible.  In  the  worst  cases  from 
25  per  cent  up  to  the  entire  crop  is  affected.  The  disease  grows 
worse  each  year  potatoes  are  grown  on  infected  soil,  but  so  far  as 
known  no  other  crop  is  attacked. 

The  seriousness  of  this  malady  may  be  judged  from  the  following 
quotations : 

A  vigorous  effort  should  be  made,  if  found  in  the  United  States,  to  eradicate 
the  trouble.  All  infected  tubers  should  be  boiled  or  burned,  and  no  more 
potatoes  should  be  planted  on  that  field  for  eight  years.  Stock  should  not  be 
allowed  to  run  over  infected  areas  and  no  part  of  any  lot  containing  diseased 
potatoes  should  be  used  for  seed  purposes.1 

It  has  been  stated  that  no  sound  potato  is  saved  from  a  crop  that  is 
attacked.  While  the  virulence  of  diseases  like  late  blight  or  early  blight  is 
more  or  less  dependent  upon  climatic  conditions,  this  disease  is  not  influenced 
in  any  known  degree  by  physical  or  mechanical  conditions.  It  may,  there- 
fore, be  considered  as  the  most  serious  pest  attacking  potatoes.  Fields  at  har- 
vest in  affected  areas  present  the  most  hopeless  appearance;  the  disease  has 
caused  the  greatest  havoc  in  all  localities  where  it  has  appeared.2 

Powdery  Scab  (Sponyospora  suMerranea) . 

Powdery  scab  has  only  recently  been  introduced  into  the  main- 
land of  the  United  States,  first  being  found  in  Maine  and  subse- 
quently in  other  New  England  States  and  more  recently  in  Oregon 
and  Washington,  and  it  is  also  known  to  exist  in  British  Columbia. 
This  disease,  which  may  be  mistaken  for  corky  or  common  scab,  is 
caused  by  the  slime  mold  Spongospora  subterranea.  There  appears 
to  be  little  indication  that  it  would  be  serious  if  introduced  here, 
the  pathologists  of  the  Eastern  States  being  of  the  opinion  that  its 
spread  will  be  largely  controlled  by  climatic  and  soil  factors.  The 
fungus  attacks  the  young  tubers  and  develops  as  they  mature  in 
the  ground.  The  center  of  infection  becomes  a  pustular-like  scab 
containing  the  spore  balls  of  the  fungus  as  a  brown  powder.     The 

1  Field,  Ethel  C.  [The  wart  disease  of  the  potato.]  U.  S.  Dept.  Agr.,  Farmers'  Bui.  489 
(1912),  p.  23. 

2  Giissow,  H.  T.  A  serious  potato  disease  occurring  iu  Newfoundland.  Canada  Dept. 
Agr.  Bui.  63    (1909),  p.  6. 


38 


BULLETIN   45,   HAWAII   EXPERIMENT   STATION. 


spots  are  as  a  rule  smaller  than  those  of  the  corky  or  common  scab, 
are  more  regularly  rounded,  and  are  characteristically  bordered  by 
the  uplifted  and  torn  skin.  A  canker  stage  also  occurs,  and  it  is 
this  stage  that  causes  the  greatest  loss.  This  scab  may  open  the 
way  to  various  storage  rots. 

Control. — Since  this  disease  occurs  in  the  coast  States,  importa- 
tions of  potatoes  from  these  should  be  carefully  inspected  for  signs  of 

this  disease,  and  all  mainland  po- 
tatoes to  be  used  for  seed  should  be 
disinfected  with  corrosive  subli- 
mate solution  as  a  precautionary 
measure.  This  solution  is  known 
to  be  only  partially  effective  in 
destroying  powdery  scab  spores 
carried  by  the  seed,  but  it  is  better 
than  formalin.  Lots  of  potatoes 
containing  suspicious  tubers  should 
not  be  planted,  and  specimens 
should  be  forwarded  to  the  station 
for  examination. 

Blackleg  (Bacillus  phytophthorus) . 

Blackleg  is  a  serious  bacterial 
disease  of  the  potato  occurring  in 
the  New  England  States,  also  in 
Washington  and  Oregon,  and  pos- 
sibly in  California,  but  not  as  yet 
observed  in  the  Hawaiian  Islands. 
This  is  a  disease  of  the  stem  and 
tuber,  and,  as  its  name  suggests,  the 
lower  part  of  the  stem  of  affected 
plants  becomes  blackened.  The 
tubers  themselves  are  destroyed  by 
a  soft  rot.  In  the  advanced  stages 
the  stem  shows  a  black  color  which 
extends  from  the  seed  piece  to 
some  distance  above  the  ground 
(fig.  7).  In  the  tubers  a  rot  often  extends  from  the  stolon,  or  else  the 
tubers  may  simply  remain  undeveloped.  The  use  of  infected  seed 
results  in  a  poor  stand,  and  the  plants  become  diseased. 

In  Europe  the  losses  are  reported  at  from  5  to  94  per  cent,  and  in 
Maine  the  most  common  losses  range  from  1  to  5  per  cent  with  occa- 
sional losses  of  50  per  cent.  The  damage  is  heaviest  on  low,  wet, 
and  poorly  drained  soils. 


Fig.  7. — A  potato  plant  affected  with 
blackleg  in  the  summer  stage,  the  result 
of  planting  diseased  tubers.  The  lower 
part  of  the  stem  is  shriveled  and  black, 
the  leaves  are  yellow  and  rolled  up- 
ward. (From  L\  S.  Dept.  Agr.,  Farm- 
ers' Bui.  544.) 


POTATO   DISEASES   IN    HAWAII.  39 

Control. — In  Maine  it  is  thought  that  the  disease  does  not  spread 
from  hill  to  hill,  and  that  it  can  be  controlled  there  by  seed  selection 
and  disinfection.  Here,  where  there  is  little  change  of  season,  it  is 
conceivable  that  the  disease  might  remain  in  the  field  from  crop  to 
crop,  and  be  much  more  serious.  The  porous,  well-drained  soils 
would  tend  to  minimize  the  damage  should  this  disease  be  introduced. 
The  liability  of  introducing  this  disease  as  well  as  others  is  added 
evidence  against  the  practice  of  using  any  imported  seed  except  that 
which  is  certified.  The  latter  should  be  planted  only  after  it  has  been 
examined  and  disinfected,  as  a  small  percentage  of  this  disease  is 
allowed  in  the  growing  fields,  the  product  of  which  may  later  be 
certified. 

Southern  Bacterial  Wilt  (Bacillus  solanaeeariim) . 

This  bacterial  disease  of  the  Irish  potato  is  prevalent  and  destruc- 
tive in  the  Atlantic  States  from  Florida  to  New  Jersey.  Tomatoes, 
eggplant,  peppers,  and  tobacco  are  also  attacked.  On  tobacco  the 
disease  is  known  as  the  Granville  wilt.  This  malady  is  most  destruc- 
tive in  dry  seasons  and  on  dry  soils.  The  affected  plants  wilt, 
recover  during  the  night,  and  wilt  again  the  next  day.  Then  they 
become  yellowed  or  blackened,  and  the  forming  tubers  are  attacked 
by  a  soft  rot,  which  is  accompanied  by  a  very  foul  odor.  Possibly 
this  disease  would  be  very  destructive  if  introduced  into  the  islands, 
but  considering  the  remoteness  of  the  infected  districts,  there  seems 
little  probability  of  this  contingency. 

Control. — Seed  from  uninfected  fields  should  be  selected  and  rota- 
tion of  crops  practiced.  Other  solanaceous  crops  should  be  avoided 
in  the  rotation.  Insects  should  be  controlled,  as  they  are  believed  to 
carry  the  wilt  from  plant  to  plant. 

Silver  Scubf   (Spondylocladdum  atrovirens). 

This  disease,  probably  recently  introduced  into  the  United  States, 
is  caused  by  the  fungus  Spondyloclculiwn  atrovirens  and  is  charac- 
terized by  a  silvery  appearance  of  the  skin,  with  minute  blackish 
patches  of  the  fungus  thereon.  It  is  not  considered  as  especially 
serious.  The  damage  to  the  crop  results  from  disfigurement,  abnor- 
mal shrinkage  and  shriveling  of  the  tubers,  and  decreased  market 
value  of  the  product. 

Control. — Seed  selection  and  disinfection  in  corrosive  sublimate 
are  advised  for  the  control  of  silver  scurf. 

Verticillium  Wilt   {VerticiUium  alhoatmm). 

Another  wilt  disease  scarcely  to  be  distinguished  from  the  Fu- 
sarium  wilt  is  caused  by  the  fungus  VerticiUium  olboatrum.  Besides 
the  potato,  this  fungus  attacks  eggplant,  tomato,  okra,  and  cotton. 


40  BULLETIN   45,   HAWAII   EXPERIMENT   STATION. 

The  disease  occurs  in  the  Atlantic  States  and  also  in  the  Pacific  Coast 
States.  In  general  the  Verticillium  can  not  be  considered  as  serious 
as  the  Fusarium  wilt,  since  only  scattered  plants  in  a  field  are 
attacked.  Once  a  field  is  infected,  however,  the  disease  becomes 
more  serious  from  year  to  year  if  cultivation  of  susceptible  crops  is 
continued. 

Control. — The  control  measures  suggested  are  the  same  as  those 
for  the  Fusarium  wilt  (p.  19).  In  the  rotation  the  following  suscep- 
tible plants  must  be  avoided:  Eggplant,  tomato  and  other  solana- 
ceous  plants,  okra.  brambles,  and  cotton. 

Leak  or  Melters   (Rhizopus  nigi'icans  and  PytTvium  debaryanum) . 

Leak  or  melters,  the  name  applied  to  a  rapid  soft  rot  of  tubers 
occurring  in  the  delta  lands  of  California,  is  a  type  of  decay  often 
causing  serious  loss  in  shipments  to  markets.  The  disease  was  at 
first  attributed  to  the  common  bread  mold  fungus  Rhizofus  nigricans, 
but  recent  investigation  has  shown  that  it  is  in  large  part  due  to  the 
fungus  Pythium  debaryanum.  These  fungi  enter  through  wounds 
in  the  tubers  made  in  digging. 

Control. — Careful  handling  in  digging  and  the  sorting  out  of  all 
bruised  tubers  previous  to  shipping  has  been  found  a  practical  means 
of  control. 

NONPARASITIC  DISEASES. 

Internal  Brown  Spot  and  Net  Necrosis. 

Brown  spots  and  streaks  in  the  flesh  of  the  potato  and  more  or  less 
widespread  net  necrosis  or  netted  brown  streaks  (vascular  threads) 
occasionally  occur  under  conditions  of  growth  that  are  not  well 
understood  by  pathologists.  Dry  weather  and  soils  possibly  deficient 
in  some  element  of  plant  food  are  thought  to  be  associated  with 
this  trouble.  As  found  on  the  mainland  it  is  sometimes  associated 
with  temperature  changes,  probably  both  during  the  maturing  period 
and  in  storage.  It  is  significant  that  neither  internal  brown  spot 
nor  net  necrosis  has  been  observed  here,  where  a  uniform  tempera- 
ture is  the  rule. 

Curly  Dwarf,  Leaf  Roll,  and  Mosaic. 

The  diseases  known  under  the  names  curly  dwarf,  leaf  roll,  and 
mosaic  are  all  imperfectly  understood  and  the  causes  are  entirely 
unknown  at  the  present  time.  The  first  two  are  inherent  troubles 
which  appear  to  be  associated  with  weakened  strains  of  potatoes. 


POTATO   DISEASES   IN    HAWAII.  41 

CONCLUSION. 

The  growing  of  Irish  potatoes  in  Hawaii,  though  an  industry  of 
considerable  importance,  has  been  largely  discouraged  in  recent 
years  by  persistent  crop  failures  and  unfavorable  marketing  condi- 
tions. It  is  desirable  that  the  islands  become  self-supporting  so  far 
as  possible  with  respect  to  this  commodity.  It  is  believed  that  the 
potato  crop  can  be  doubled  without  any  increase  of  acreage  by  the 
adoption  of  modern  agricultural  methods. 

Crop  failures,  which  have  been  found  to  be  a  persistent  source  of 
loss,  are  brought  about  by  the  following  factors:  (1)  The  use  of  poor 
seed,  (2)  continuous  cropping  (as  many  as  three  or  four  crops  a  year 
with  no  systematic  rotation  for  a  half  century  or  more),  (3)  diseases 
and  insect  pests,  and  (4)  unsatisfactory  soil  conditions.  Fundamen- 
tally the  diseases  have  been  the  most  important  factor,  with  the  other 
mentioned  factors  as  contributing  causes. 

The  diseases,  so  far  as  they  have  come  to  the  attention  of  the  writer, 
have  been  determined,  and  recommendations  have  been  made  for  their 
control,  both  through  the  improvement  of  cultural  practices  and 
through  special  methods  where  required.  Certain  diseases  which  are 
destructive  in  other  localities  but  have  not  yet  been  found  here  are 
described  in  order  that  their  introduction  may  be  the  better  guarded 
against. 

The  following  diseases  and  pests  of  the  Irish  potato  have  been 
found  to  occur  in  Hawaii:  Fusarium  wilt  (Fusarium  oxysporum), 
late  blight  (Phytophthora  infestans),  early  blight  (Altemaria 
solani),  black  scurf  and  rosette  (Rhizoctonia  solani),  Sclerotium 
wilt  (Sclerotium  rolfsii),  common  or  corky  scab  (Actinomyces 
chromo genus) ,  tuber  rots  (Phytophthom  infestans,  Fusarium  oxy- 
sporum,  F.  radicicola,  and  F.  coeruleum),  tuber  moth  (Phthorimaea 
operculella) ,  cutworms  (and  a  similar  leaf-eating  worm  locally  called 
"poko"),  mite  disease  (an  unidentified  form  of  Tetranychidae) , 
nematodes  (Heterodera  sp.) ;  and  the  nonparasitic  troubles  sunscald, 
sunburn  of  tubers,  leaf  tipburn,  hollow  potato,  pronged  potatoes, 
arsenical  injury. 

The  universal  adoption  of  the  practices  of  seed  selection  and  dis- 
infection, crop  rotation,  and  spraying  with  Bordeaux  mixture  are 
recommended  for  the  general  improvement  of  the  potato  situation. 
Only  through  the  use  of  healthy,  vigorous  seed  and  fertile,  disease- 
free  soils,  maintained  through  crop  rotation,  can  healthy  crops  be 
secured.  When  once  healthy  plants,  in  other  words,  plants  worth 
saving,  are  assured,  the  value  of  Bordeaux  mixture  in  preventing 
late  blight,  the  most  serious  potato  disease  in  Hawaii,  will  be  more 
readily  seen,  and  under  improved  cultural  conditions  spraying  will 


42  BULLETIN   45,    HAWAII   EXPERIMENT  STATION. 

more  readily  be  adopted  as  a  universal  practice.  At  the  present  time 
some  of  the  growers  are  taking  up  spraying  and  other  crop  improve- 
ment measures  with  commendable  zeal,  and  sprayed  plats  have  shown 
increases  of  from  50  to  200  per  cent  over  unsprayed  plats.  In  con- 
trolling the  late  blight,  a  locally  developed  variety  appears  to  offer 
considerable  promise  in  resisting  foliage  injury. 

The  improvement  of  the  potato  crops  can  be  permanently  brought 
about  only  through  a  realization  of  the  importance  of  the  several 
factors  involved  in  the  crop  failures,  and  especially  the  diseases  and 
pests  must  be  prevented.  This  can  be  accomplished  only  through  the 
use  of  good  seed,  fertile,  disease-free  soil,  and  spraying,  combined 
with  the  best  cultural  methods. 

o 


plorida 


